sp1: increased fps - decreased smoothness

Before SP1, FSX was very enjoyable on my system - even with 20fps or a bit lower (what I got on very dense areas). After a clean install of fsx-->sp1, I got a "performance" boost from 30-50% in fps - but smoothness is gone.Now I have continuously so-called micro stutters (sometimes a bigger one) @ 20 or 30 fps(capped) with no obvious fps drop down. I tried several tweaks I knew or saw in the forums. But most of them deal with the 'real' stutters when fps goes down for a more or less short moment. In my case this never happens. The stuttering seems to be independant from fsx settings because keeping the sliders very conservative does not help at all.appreciate your helpkind regards, nemomy specs: C2DE6700@3500, 4G, 7900GTX w/93.71 or 101.09 @ 1680x1050, 2WDRaptor (one for FSX), XPSP2x64, FSXSP1, no addons, no tweaks.