Wireless Advise/help please

I'll start with my setup. I have a motorola cable modem upstairs in the corner of the house. My cable company won't let me move it without calling a technitican and blah. So I have a WRT310N linksys router connected to it. Signal downstairs is weak and speeds that should be atleast 54mbs (g) if not N speeds are around 11mbs (b). So i recently setup a powerline network PLK300 linksys so that i could connect an old router as an access point downstairs for better speed and signal. The old router is a WRT54Gx2 linksys. I have the setup and speeds downstairs are 54mbs and are far faster and stronger than before.

After setting this up I got greedy. I figured that my WRT310N wasn't really being used to its potential because we usually only use the internet downstairs. And because it is a faster and more powerful router i figured i could just configure it as an access point and switch the two routers. So i did. What happened at first led to triple the speeds i was getting out of the WRT54Gx2 as an acces point, but the signal was getting dropped at first around every ten minutes then every five then nearly every minute. The problem was that the WRT54Gx2 (now hooked up to the cable and cat5'd to the powerline network) was restarting itself every few minutes for no apparent reason (it didn't do this while i was using it as an AP and isn't doing this now as i'm using it as an AP). A 270mbs connection is pointless if the signal is dropped every few minutes, so i switched them back to thier previous settings and places.

I upgraded both of the routers firmware before the switch and they are both working properly now. The problem is i'm still greedy and I want that N router as my access point downstairs. Any ideas or reasons as to why my WRT54Gx2 would restart when connected to a modem but not when used as an access point through a powerline network? Any ideas for how i could fix it? (there is no third party firmware for the WRt54Gx2 as far as i know) Anything would be extremly appreciated. Thanks.

(ps i reset the WRT54gx2 to its original settings before the switch but i didn't reset the WRT310N and i have configuration settings for both saved)