You have a vacuum leak somewhere in the heater control system, most common is a cracked line that feeds the entire unit, it comes through the firewall off of the engine intake manifold, then the switching valve itself can leak and then finally the vacuum servos. These can be a real bugger to find, so be patient if your doing this yourself as it can get bothersome.
Check for the vacuum canister if vehicle is so equipped. It may look like a coffee can or be round with vacuum hoses going to it. They can develop a leak and not hold vacuum. When the vehicle accelerates and draws vacuum, the canister supplies vacuum to those components needing them during those times.
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