Heater or A/C will blow normal when stopped or when slowing down

Welcome to 2CarPros.

Yes, it is a vacuum leak. The engine vacuum is supplied to the controller on the dash. When you load the engine, the vacuum in the engine drops, so if there is a leak, the mode actuator can't select where air flow should go.

I attached pics below of the system and the vacuum lines. You will need to inspect them under the dash. Often times, you will hear a hissing sound when the engine is running. Also, most times the leak isn't in the vehicle, but rather under the hood. The hard plastic hoses they use get brittle and break or crack.

Here is a link that explains how to find a vacuum leak. This is mostly for under the hood.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know if I can help.

Take care,
Joe