Have you replaced M/T Oil recently? Sometimes the oil is so dirty & thick because its been in there working a long time & if you drain & put in new oil, the transmission will start messing up because the new oil is thinner. Know what i mean? Like in an automatic trans, if its slipping & you put new oil there's a 50% chance itll run fine & 50% chance it will mess up completely. Well thats what a tech told me.