The description you gave is strange, if disconnecting the AT fixes the problem then it is probably not the GSmini. GSmini would be the obvious answer, when on approach did you disconnect an reconnect AT? The AT system can be quite lazy so if the OFF/ON of the AT was done in the latter portion of an approach it might be that it simply hasn't redone calculations and made the input yet to achieve the targeted GSmini speed speed. Next approach maybe try the same OFF/ON selection but at an earlier time on the approach and see if it is still carrying the extra speed or not. If it does reset itself still like you have mentioned above I would ask your airlines tech pilot for some clarification.