There is another possibility.
I had a 92 Chevy lumina. There was a vacuume redo our on the car and the plastic tube that ran to it ran under the battery tray. For some reason 3 times in the 10 years I drove it the hose wore through and developed a leak. That caused the vacuume operated heater controls to not work correctly.
You might have a similar issue. The increased RPM changing the vacuume signal just enough to open a valve or move a mixing door.