737 Weird Behaviour and other things

The plane's been doing some weird things after flying around for several days. These problems seem to arise after I installed several liveries.(1) I cannot load up a plane from the main menu. When I get in FS02 I get a darkened screen in the VC with a small subscreen to the left. I then get a MS error message which gives me the option of going back to the desktop or FS02. I can, however, load the plane from a cold and dark which I had saved previously. Very curious. I reinstalled PMDG 737 (thus a new aircraft config. file with no liveries) but the problem persists. I am running nothing but MyTraffric and FSNavigator in the background. These latter are not the culprits as I was running them before I started adding the liveries and it was running ok.(2) After starting from a cold and dark noted above, programming the FMC and taking off, when I press the "g" key the gear doesn't go up. So I click the gear lever in the cockpit and the lever goes up showing the gear has gone up. But when I spot view the plane the gear remains down. Now, this latter occurred until I reloaded PMDG 737. The problem disappeared. I mention it because I was hoping that (1) above would disappear also because of the reloading of PMDG. (1) has not disappeared but (2) has.(3) When getting to the TOD noted on the FMC I manually change the Altitude to the next lower altitude. No response. I then have to manually program the descent. As well, I noticed that when you're going to a target altitude at waypoint X the altitude is concurrently changing at the other waypoints XYZ. Should not the altitudes remain stable until I get to those subsequent waypoints? Otherwise which target altitude would I know is appropriate for each waypoint?LeonardP4, 2.8 Ghz, 533 mhz FSB, 1 gig DDR ram, Radeon 9700TX 128 mb ram, windows XP home.