I have a 2004 Chevy Tahoe. At the beginning of the year my speedometer started acting up. When we first start the truck and take off for the first 10-20 minutes maybe or few miles the speed is correct and it is moving according to the speed I am going but then will slowing drop even if I am going a faster speed. Then after that will drop to 0 and will not move any longer. What can cause this? My transmission went out and when I took it to get fixed the guy said it was because of the speedometer not working so the transmission couldn't tell what gear I was in and caused it to mess up. They are stating the speedometer messed up because of the ignition which gets stuck at times when turning on but with a slick flick backwards it goes into the right place and everything turns on as normal. The other gages are working fine I don't see any problem with the gas or oil gage only the speedometer.

Replace the Vehicle Speed Sensor (VSS), which provides speed information to the speedometer and transmission. It should be a simple problem to solve. Afterwards, clear any codes with a code reader.

I'm sorry to say but your particular vehicle is prone to Instrument Panel Cluster (IPC) failures due to it's design.
You may have some satisfaction if you contact your local GM dealer but you may need to be assertive, too.
The problem is believed to be the cluster electronics failing prematurely.
If the vehicle meets certain criteria, the dealer may be able to help share the cost of repair.