Over heating up a big hill I would deffinatly say blocked radiator, you may be able to just pull it out and give the core a good hose out to get all the bugs and mud out of it.
I do this about once or twice a year depending on how much mud I go through as it does get a bit clogged.
Just make sure when you do do it you dont use a pressure washer so you dont fold the fins over and you use plenty of water, I use dam water at work for this as it just keep recyling.
As far as the loss of power......has anyone mentioned the throttle cable, make sure your getting full throttle as even being a little bit out of adjustment will make a huge difference.
As far as economy goes, Kick down cable adjustment, if it is broken then the clutches can slip losing power (this also means you may be up for a new trans), if it is too tight then it will kick down to early and will eventually break the cable due to it runningout of throw and if too loose it wont kick down early enough and you will be lugging the thing up every hill you come accross.
They do wear the pin in the kick down cable as well as the hole and the cable stretch so they do require regulare adjustment checks.
Im not sure what the propper adjustment (off hand) is but I set mine by taking up the slack and then going a bit more.
All of these should only cost you some time unless you find broken stuff along the way.
Im with Dave on the cruel and unusuall punishment of doing a stall test but check your kickdown cable first and to the stall test last as you dont want to be stuffing your trans because of a broken kickdown cable but the torque converters can fail on any auto.
