737 after replay

Strange AP behavior after repay of takeoff. When I return to the cockpit the AT is off AP is off as well VNAV. When I try to re-engage VNAV it will not engage. I get one of two messages on the FMC. CHECK MPC ALTITUDE or UNACHIEVABLE ALT. For the rest of the flight VNAV is unusable. Has anyone experianced this?One more thing, I've turned of AA and AF,(in flight} in cockpit fps are 18-23 VC, a little lower, yet in spot I can't get above 10fps usually 5-7fps, With or without clds, while most payware, and I have many, get 18-26fps in spot.Dave FisherCYYZP4 2.8 GB 478P 533 MHZ CPUP4P800S Asus Motherboard768 DDRGeforce Ti 4200/128Maxtor 80 Gig ATA 133 HD x2http://www.precisionmanuals.com/images/forum/ng_driver.jpg