Climate Control not working

[Bamataz15]

I have a 1999 F150 and the climate control system isn't working correctly. During cold weather I cannot get hot air to blow and during warm weather I get slightly cool air until the engine is put under stress, i.e. climbing a hill, and then the air turns very hot. Changing the temp gauge will not affect the temperature of the air, only stressing the engine seems to affect it. Dealer says they haven't heard anything like this before but will to check it out for a few hundred$$ can anyone here offer any suggestions?