FSX or my PC or what?

For testing different slider settings and tweaks in FSX, I use the t/o from rwy 34R in SEA. Then I make a short flight towards downtown Seattle, watching the effect from on the scenery, the fps etc.Today I noticed after a short run along the runway that those highest buildings downtown just "jump" on top of each other. Approaching the river (the blue stream) closer to the harbour, the buildings "jump" back to their proper place.I couldn't get rid of the problem, so I made a total re-installation of FSX - with the default settings.Still the same "jumping" exist with absolutely nothing tweaked!!!I have always been watching the scenery (trees/lower buildings) during these tests. This problem might have been there all the time - but I haven't noticed it until today!Is this related to FSX or is it my PC (HP Pavilion; P4-3,4 MHz; 2 GB HDRAM; GeForce 6800)?Does this happen to you, or is it just to me?Haraldh F.