My suggestion would be carb ice. The A65 is very prone to carb ice, and our A65 fitted aircraft will ice up if idling over damp grass. In both cases mentioned, the engine will have been at low power prior to the throttle being opened. I normally taxi out to the runway with carb heat applied and only push it to "cold" after fully opening the throttle.
I expect you already have carb heat applied in your descent, so I wonder if the carb heat is working correctly?
Do report back to us when you solve it.