Hi again Sonny,
I've been thinking about this issue these days and frankly I can't say I have a clue about what is happening there. I think I saw something similar to that at some point while I was developing the UI back in the day, but the interface never stopped doing its job, even if the button blinked .
To me it doesn't seem to be related to that 7800GT, but who knows? If the GPU were the problem i guess you would have glitches somewhere else.
Even if the button disappears (its image, i mean) the button action should remain available in that spot of the screen, so even if you click there while not seeing any button the action ("jump to race") should be triggered.
Far from being the best, but I guess that if you can't play like this the best would be removing the custom UI and use rFactor's stock one. You'll lose not only the eye candy but the functionality additions we added to it (which are specially important when it comes to select your opponents) but I can't see a way of fixing your problem at this point.