In case your system has speedstep / AMD64 of newer generations, then most likely a loop that raises CPU usage from 30 - 80 might be a reason, as the stepping is changing that drastically that aboves value gets totally useless.
Do a high speed loop (1000000 steps and measuring in that time) at the beginning and use that to calculate the steps but do not use anything more precise than ms on intelligent CPUs in realtime ... that just won't work without strange and (for us notebook users) annoying glitches ... Its no fun if we get annoyed by "please disable speedstep to run this blabla" as some games do ^^
Do a high speed loop (1000000 steps and measuring in that time) at the beginning and use that to calculate the steps but do not use anything more precise than ms on intelligent CPUs in realtime ... that just won't work without strange and (for us notebook users) annoying glitches ... Its no fun if we get annoyed by "please disable speedstep to run this blabla" as some games do ^^