Cpu turns on but no display ?

Apart from a failing GPU it can be PSU, bad connections and etc. Follow this guide and report back http://www.tomshardware.com/forum/261145-31-perform-ste...

I just cleared the bios by battery & reinstalled the vid card & ram & it turned on , ive done it before, sometimes it works & the pc works fine for a few days but then suddenly the problem comes back...its a strange & difficult problem, I would be very greatful if solution is provided.

If it's a RAM issue, try to clean the part which is connected to the motherboard with a pencil eraser. Simple yet effective.

Believe it or not, I just did it before I post this comment.  

If it's still not resolved yet, try using a single piece of RAM first and see how it turns out. If using 1 stick of RAM resolves your issue, while using the other recreates it, then you've found the cause of your problem.  

@malmental

No beeps at all when it happens, only the fans rotate at a reduced speed.

Ok so your not failing your POST then. Have you tried to reseat your video card, have you tried booting in safe mode, are there newer drivers out there. If i was you the first step would be to check all connections, try another monitor (if available), and then probably try another video card or onboard.

Make sure your CPU fan is securely fastened if its loose almost wobbling then thats why, if it isn't that your PSU isn't giving enough power to your board

I had tried different gpu, onboard video, different monitor....but when it goes out nothing can be done by me.. & after a month or so when i just once again remove the mobo battery to reset the bios, it just starts again , works for a few days & then again the same condition returns....I am very disappointed with my pc. Can any one plz figure it out.

You have some kind of automatic overclocking / optimizing setting enabled in BIOS?
Resetting the BIOS back to defaults seems to help so there could be automatic changes there that are creating issues.

The only thng i changed in bios is the cpu vcore voltage @ 1.375v for phenom 2 965 C3 revision, which if left to auto goes 1.47v which i think is really high, is 1.375 v too low for this processor?

http://www.guru3d.com/article/phenom-ii-x4-965-be-revis...

I'm thinking, judging by the troubleshooting steps and the original problem, it could be the PSU?

You didn't change any jumpers on the board to do with voltages

@Jonathern
no man..

@ Every one , today I turned on the PC & guess...same problem again   I am really upset with my pc , could it be a defective mobo , plz tell I dont have knowlege in this regard.

Well dude people here are trying to figure out every possible solution, I can only assume your graphics card is defective somewhere on that card there is a fault, Possibly one of the RAMDAC's is torched.

I also installed a 4650 & the onboard hd 4200 also are giving the same problem..

One thing that seems very strange to me is that when i start it after some time, it starts & works for a few days ,then again the problem starts, like a cycle.

mmmmmmmm hi there it could be a busted capacitor/s in your mobo,i had this problem just two days ago from a client..... what i did is replaced the busted caps with the same voltage and microfarad... some soldering skills required... hope this help

How to identift the busted capacitor/s ? And if the capacitor is busted how come it starts sometimes?

Look at the top of all capacitors on the board they are supposed to be flat not bulging.

Try clean out the PCI slot and see if there is any change

the same problem i faced its wire issue.

@dream2005 plz share how u fixed it.

definately motherboard my friends have the same problem try changing the psu first though as you have said the fans run at a reduced speed that indicates a psu problem
and if that dosent help go bet a new MOBO thats definately ought to solve the problem

@ vada , sir , changing parts one by one - isn't it the last resort?

somewhat my other comp had the same issue it was a mobo problem so i thre it out and got a new one and it worked

It's a motherboard problem - Never solder anything it will do more harm than good.

JUST go and change the MOBO and all will be fine