Possible motherboard problem - guidance required

Hello,
I have had some quirks with my PC for a while now and have been told its either a motherboard or memory fault. Id like some guidance from people who aren't trying to sell me stuff to see if this is the most likely cause.

Firstly specs -

E6600 Core 2 Duo (2.4 GHz)
Decent heatsink - arctic cooler 7 IIRC?
EVGA motherboard with nForce 650 ultra chipset
4 Gb rem - 4 x identical modules of Geil Ultra 4-4-4-4-12 IIRC
Vista home premium 64bit
Nvidia 8800GT

PC was bought in parts and assembled by myself in 2007, only hardware tweaks in that time are upgrading from a 8600GT and doubling up from 2 gigs of ram. I bought the PC to run games of the time and future - (ie. until about nowish) since it overclocks easily to 3 GHz for the rare occasions when i want to play crysis. All clock speeds are standard now and almost always too.

My problem is this - i need to wirelessly access my router for internet access and i'm using a netgear WPN111 USB type connector. Now, i can generally browse and shop for example fine. However, if i try to access streaming audio, streaming video, large file downloads, online gaming etc then a something strange happens to my connection. It seems to partially uninstall itself. As in, the netgear icon which is red. yellow or green disappears completely. After it chooses to do this nothing apart from full uninstallation and reinstallation of the USB adaptor will allow me to access the internet again. Restarts do nothing, trying to reopen the netgear software causes it to open for a fraction of a second and dissappear again each time its clicked on. These symptoms have been going on for the last year or so and its starting to drive me mad! Im so sick of a ~180Mb driver download being cancelled just before it finishes for no apparent reason, sick of not being able to play online games for more than 15 mins (never finished a game yet!!) and sick of using youtube and the like to cause my internet access to be lost repeatedly!

I also have another netgear wireless USB adaptor from a while back - a WG111v2 - and i can install it and although all appears to be working (the lights on the device indicate it is and the icons on the computer indicate that it is) but no internet access is possible with the same, generic, false and pointless suggestions for lack of connectivity.

However, having been so sick of sporadic internet access for so long i searched google for my symptoms and it appears that im not alone with having problems with netgear wireless adaptors. I went to the IT shop across the road looking for a decent replacement assuming that the USB adaptor was at fault. I bought a D-Link DWA547 PCI card type on the advice on the shop owner and also from searching from reviews on the internet. I installed the card and the installation wouldnt pass 95% and then everything froze. I restarted and it seemed to be installed. As far as the software was concerned it was fully installed and workin. I could detect and link to my routers network but couldn't access the internet - exactly as the WG111v2 did before. Uninstallation and reinstallation of the PCI card adaptor caused many varied, random faults including freezing, unstability, boot failures, BSODs etc so i have uninstalled the card and am back using my sporadic WPN111 again. Although the D-link card is physically removed from the computer the software doesn't seem to fully uninstall?

The guy in the shop informed me that for things to be uninstalling itself it HAS to be a motherboard or memory problem - is this correct?

To aid fault finding -

1 - 3 different wireless adaptors surely cannot all be faulty in similar ways so i am assuming that these are fine.
2 - These symptoms happen even with a completely clean format and installation of vista so it cannot be a software or virus issue
3 - All programs, drivers, windows etc are up to date and patched etc. Windows is genuine too
4 - My system is rock stable with many demanding games with most of the settings turned on. These problems seem confined purely to networking issues although i can connect directly by cable to the router by moving the computer to it although this is no way a workable solution to have my computer on the livingroom floor and also im not trailing a long cable from the livingroom to my PC!
5 - I get these symptoms using 2 different routers in 2 different houses using 2 different ISPs and connection signal is also not the issue here. Connection is flawless when it chooses to work, unworkable when it chooses not to.
6 - Although i have used 3 different wireless adaptors i get these symptoms with only one installed at a time - theres no conflict issue between them.
7- I have 4 identical Geil 1GB DDR2 modules, ive tried using only 2 gigs by swapping every combination of modules in and out as well as using 4 gbs and the symptoms are always the same. Thus, i do not believe my memory is as fault.
8 - However, when installing vista as a clean install i must remove any two memory modules and install with only 2 gigs. If i install vista with 4 GBs i get BSODs almost at the end of installation. Once i use 2 gigs to install and then download a few updates i can use all 4 GB no problems - is this quirk relevent to the whole networking issue??

Part of me wants to buy a new motherboard and stick it in but its nearly £100 for one similar to what i have, but if im spending more than a few quid then i may as well get a decent board with DDR3 compatibility, (which means new memory and processor, both of which are fine!) - and i'd be cheaper just getting a cheap laptop for internet access it thats the case??

Thanks in advance for any help and pointers you can give me guys. I know that this post is mega long-winded but i believe all the info is relevent to getting suggestions/ideas/solutions. If you require any more info then please do not hesitate to ask.

Regards,

Mike