Have you ever flown a small plane at it's max altitude?
I took a Cessna 150 up to it's max altitude, and it got real squirelly.
I didn't stay long, I was at full throttle, going along at a nice clip, and the stall warning horn was going on and off the whole 5 minutes I was there.
This may or may not have anything to do with it, but personally, I don't have the desire to do it in any other aeroplane to find out.