Every time I'm on the interstate and I'm going downhill the speedometer goes haywire and starts bouncing on me. By bouncing i mean it'll show that I'm going 70 at the beginning of the hill and while I let off the gas my speedometer goes to 20 mph.. It'll go back up if I either hit the brake or hit the gas. And once it does this my cruise control kicks off. Any thoughts on what it could be?
I'm thinking that may be a problem with the needle motor in the IPC. Since the vehicle speed input is a critical input to the ECM & you are not complaining about any drive-ability issues (except the cruise control disabling), I suspect that the VSS input to the ECM is true. Now if you were also complaining about trany shifting & converter engagement issues I'd say it was definitely the VSS. Maybe you aren't seeing these other issues due to the problem being so intermittent. Easy way to check is with a scan tool that can read real-time sensor data. A simple code reader can't do this. Any repair shop has this capability. You would want to check the VSS value during an intermittent event...i.e.-while recreating the issue during a drive. Remember, even if a scan tool reading shows a faulty speed sensor signal, you will still need to check wiring and connectors before condemning the VSS.
If it is the needle motor, you are looking at a new IPC unless you are a very resourceful electronic tech. IIRC, the dealer gets in the $400 neighborhood for them. IIRC you may be able to get them directly from GM's supplier, a company called Specmo, for somewhat less.
If it's the VSS, they are less than $25.
I would first want to know what the real-time speed sensor data shows before doing anything. Since the ECM receives the speed sensor data & passes it to the BCM which in turn passes it to the IPC, it's a complex chain of events & I could be wrong about the IPC as the culprit. There are rarely easy answers on computer-controlled cars.