Odd Aircraft behavior while minimized

I've recently built a new system and I'm experiencing a strange phenomenon in FS9 if I "Alt+Tab" back to my desktop while in flight. I'm flying on Vatsim using Squawkbox, and if I alt+tab out to do something else, my aircraft is apparently behaving erratically in my absence. I'll return to FS9 to find my aircraft 1000 feet or more below my cruise altitude descending at a good rate, and usually quite a bit slower than I left it. So far this has occurred with the PSS 757 and LDS 767. A quick glance at my MCP reveals that the autopilot is in the same state I left it, in VNAV and LNAV. The MCP altitude and speed are unchanged, so that would rule out some keystrokes or mouse movement outside FS9 having an effect on it. Immediately upon my return, the autopilot recovers the aircraft and gets it back to its assigned altitude and speed. If I stay in FS9 the entire flight everything is fine. Checking my history on vataware.com reveals porpoising of my altitude while I'm away. Luckily all these episodes have occurred while in uncontrolled airspace, so no one has had a chance to yell at me yet. This one really has me baffled and annoyed, as it never happened on my old PC. I don't really want to have to stare at my flight deck the entire trip, especially if I need to pull up a chart or something. Anyone encountered a problem like this before?For the record, I'm running Windows 7 Home Premium 64-bit.AMD 965 Black EditionNvidia 280 GTX4GB RAM