2002 Ford F-150 Speedometer - Transmission problem

While driving 70 mph, the speedometer dropped to 45 (it bounced to 70 several times). I tried my cruise control but it didn’t hold speed at 70 mph (it may have held speed at 45 mph?). When I turned the cruise off, the OD light began blinking. I tried the OD button and it worked fine. I exited the freeway and stopped. When I took off, the light stopped blinking and everything seemed normal. I went to the Ford dealer after work and had the codes checked. It came up with P0500 (VSS malfunction), P0721 (OSS range/performance) & P1728 (Transmission slip error). They recommended changing both sensors. They ordered the parts and I returned a week later to have them replaced. During that week the speedometer worked fine when I was on the freeway, but I noticed the speedometer would sometimes drop from 20 mph to 0 when slowing or stopping. If I had to accelerate when the speedometer was at 0, the transmission would only be in first gear. I also noticed that if I was stopping real slowly, the speedometer would drop at 20 mph (3-2 shift) and then come back up at 10 mph (2-1 shift). The day I went in to have the sensors changed, when leaving for work and after work, the speedometer did not work from a dead stop and the transmission was only in first gear until the speedometer started working (within a minute). Changing both OSS and VSS sensors and clearing the codes did not correct the problem. I went back to the dealer a week later to have it re-diagnosed. The technician was not sure what was causing the problem. His laptop did indicate something happening with the 3-2 downshift. I was told it could possibly be one of four things: GEM module, wiring, transmission problem or something wrong with the ignition coils. I believe he told me it was showing the P0500 code. I am waiting for them to investigate the problem. The cruise control works fine and I’ve experienced more problems from a dead stop (usually after starting the vehicle). Any suggestions would be greatly appreciated!