first... do an upline with power and bang the rudder.... see if it does what you describe.... (Do seperate upline verticals testing left rudder and then right rudder)

then....
try doing a downline at idle power and see if the phenomonon still exists...

if it does this on an upline with power, but not on a downline without power.. prolly it is a prop/P-Factor dealy-bob.. otherwise something else going on....