Why does the a/c blow cold until ,I come to a hill and then switches and blows heat. I know this must be a vacuum problem but don't know what to look for.Any help is appreciated.

The control mechanism used to move or hold you airflow diveter doors is a small diaphragamvacuum motor. Usually one for each door. When you increase the load on the engine, and ther is wear on the postin rings, it can lose vacuum in the intake manifold. There is a vacuum canister(reservoir) in the engine compartment of the vehicle. It has enough volume to temporarily keep vacuum levels in the control system during loss of engine vacuum. Sounds like the internal check valve in the reservoir is leaking-common problems with age-- or you have a leak in the control system that allows loss of vacuum during periods of low engine vacuum. Check for loose vacuum connection at the rear of your control head assembly in the dash. then lool for loose fittings on any vacuum motors. Visually inspect all hoses for cracks or obvious deterioration. Hard lines crack with age and the rubber type lines can get soft and porous or develop cracks. Mightr want to check for manifold vacuum at the engine port when engine is idling. If you can get a vacuum gage, the engine should rpoduce a vacuum of approximately 14 to 18 inches of Hg at idle( throttle fully closed) and show less as the throttle is opened.