Please Help, FMC problem

Greetings,I've had PIC for quite some time and have always enjoyed consistent performance with it. The last couple of days, I've had something strange happen with my flight plans that I import from FSNav. I've been importing them for some time now and this is new. I'll make a flight plan in FSNav and export it to PIC, so far, so good. I go through the usual setup procedures after pulling up the flight plan: Verify legs, set dep. and arr. runways, go to performance page for take off weights, cruise altitude, and the take off page to set flaps, etc. Up to this point, all looks fine. I activate the flight plan and all is still OK. the legs, speeds, and altitudes all look good. I take off and everything looks fine at the FMC. The legs, speeds, and altitudes still look the same as when I entered them. As I approach my cruise altitude, I notice that the little red t/c (top of climb) starts moving forward on the ND, even though I'm now at the cruise altitude and top of climb. At this point, the altitudes on the FMC start changing to higher altitudes. The VNAV button is not on anymore and I can no longer activate it. The ALT hold button is on and the only way to turn it off is to activate the VS button. When I start to descend to a waypoint at altitude say, 16000 ft., the altitude starts going up as I approach it. The altitudes for all the rest of the waypoints start going up too. They change as frequently as the miles do.Can anyone out there shed some light as to what this may be, I don't have a clue as it never did this before. I tried a reinstall and it still does the same thing. After the reinstall, the t/c still moved forward as I approached it and the VNAV button went out again, but the altitudes stayed the same this time, weird. I was also unable to activate the VNAV button once again. Thank you for any help you can provide.Sincerely,Hans [email protected]