Engine has new head gasket-water pump-timing belt-new radiator. Now I have no power going up hills. Transmission does not shift in sync. Cruise control shuts off going up hills. Check engine light comes on 10 to 20 miles after resetting it. Where do I start?
Pull the codes. That's where you start. Anything else in my post is purely speculation..
You had the timing belt replaced. The Forester engine has a peculiarly long timing belt, because it has to synchronize the valves on each side of the engine. If the mechanic hasn't done many Subaru engines (and even if they have, this can happen), they may have installed the belt one-tooth off. That would raise codes P0302 and P0304, or P0301 and P0303, depending on which side of the engine is off. It will change the timing of the engine, losing power, and will set a check engine light, which will prevent cruise control from working. You generally also hear the misfire as well, though.
If these are the codes you're getting, then the car needs to go back to the mechanic who worked on it - there's nothing you can do to adjust it apart from taking the timing belt off again and resetting it.
As I said - this is speculation until we get the actual codes.