While not on a dash-8, i've had this exact problem before. What I've found is that it is actually the auto pilot roll servo not "letting go". When this has happened I've found that a very abrubt medium deflection input to either left or right will kick the roll control back to the yoke. You'll clearly feel the 'kick' free. If you're quick to counter correct this won't upset the folks in the back. If this happens to you again try it.