MacBookPro5,1 and NVidia 9600M GT problems

I own a MacBookPro5,1 which as an NVidia 9600M GT with 512 MB RAM on board. I bought it in April, 2009 and it worked fine until March 2011, even with Windows 7 64 bit, which I use for gaming.

In March 2011 the problems started and they last until now: After about 20 minutes of intensive use of the 3d graphics (Google Earth, any game), I see a massive breakdown of framerate for about 5 - 10 seconds (this varies), and then the framerate gets back to normal. The framerate gets down to 1 frame per second or even less. This is completely indenpendend from the program I use or the graphics settings within the program. After the first fps breakdown this repeats nearly every ten minutes.

I also monitored the temperature of the GPU with GPU-Z under Windows 7, it is at a constant level of 80°C, which is ok. I know the GPU changes the clock at about 105°C, but this is far away from that.

I had in mind, that there some problems with NVidia notebook GPUs, so I googled and found some people telling something about problems with the tin-solder used for mounting the GPU, but nothing official. My question: Does anyone know if this is a known and maybe common problem with this GPU type, or does it seem to be a problem with just my GPU? Any suggestions?

Greetings,

Marcus