gpu temperature, cpu fan speed

Modell-Identifizierung: iMac5,1

Prozessortyp: Intel Core 2 Duo

Prozessorgeschwindigkeit: 2.16 GHz

Anzahl der Prozessoren: 1

Gesamtzahl der Kerne: 2

L2-Cache (pro Prozessor): 4 MB

Speicher: 2 GB

Busgeschwindigkeit: 667 MHz

Boot-ROM-Version: IM51.0090.B09

SMC Version: 1.9f4

Seriennummer: W86372RPVUW

haven't used my imac for about 3 month. now as i came back home, the cpu fan regularly starts going crazy (up to 3000rpm) when watching videos (no matter if flash or other formats), or running xplane flight simulator .. in short gpu intensive operations. also does it when running windows, so it seems not to be a software related problem. however i never noticed the fan going that fast and noisy before. on istat all temps look normal around 30-45°C, except for the gpu temp, which raises up to 85°C, don't have any comparison if it did so earlier though.

however, i opened and cleaned the fans of very little dust, did nvram, pram and smc reset .. no effect. beside the perfect solution of this problem) i'd like to know some other stuff which came up in this context.

(1) i was not brave enough to actually remove the heat sinks. has anyone already tried to replace the cooling paste? does it have big effects on such a problem? i saw an other post suggesting this idea, but without any feedback.

(2) i have been running apple hardware test, which interrupts telling this error code: "4SNS / 1 / 40000001:VDOR", which is a malfunction of a sensor on the logic board. however i couldn't figure aout which sensor exactly it is about. any ideas?

(3) furthermore this error already was there looong time ago, when there wasn't any noticable problem with the imac, so i assume that it is not really related to the fan problem. therefore i'd like to know if there are some other malfunctions but: how can i force the hardware test to proceed? it always terminates with the sensor error!

thanks for any hints. imac is still working fine without graphical errors, but is gets pretty annoying.