I believe it's a realistic behavior for an unaugumented aircraft, traveling at speeds in excess of say 450 knots, to be very responsive in pitch resulting in high instantaneous climb/descent rates.
When flying in an IFR like fashion I recommend keeping the speed below 400 IAS (or maybe lower) when cursing, if constant altitude is a recuirement. Remember the higher you go the higher TAS you will get from a given IAS, so 300kts at 20 000 might still be an acceptable speed but the aircraft is probably less sensitive in pitch.
I have no idea regarding the drag though!