My P7 and PP(base) on Vista Ultimate 64bit started this week dropping rendering threads.

Usually they start normally processing lights with single thread.
Then starts utilizing all 4 cores as it start rendering, but in middle of render it drops the load to 75% then to 50% and finally to 25% or near 0%.

This happens both with and without external render process.
And Pro is using 64bit FFrenderer.

I have to recheck when I get home if my Vue does the same, so it could indicate temperature problem under heavy load. But if I remember correctly when poser started doing this Vue has still rendered with all cores thru the whole render.

Really annoying as it take quite a while longer to render with singe core :(
No changes in system (except there might be some MS patches earlier this week that I have to check). Couple addtions to runtime, but this happens even I'm not using those or if I just load some old scene which rendered ok previously.

Any other ideas why this is happening?