Change the gear oil in the tranny - I run Royal Purple in Geronimo and that solved the problems I was having like that. If it doesn't help, then the syncros are going, or your clutch isn't fully disengaging when you shift - you need to have 3/4 of an inch free play in the pedal measured at the top of the pedal where the adjusting screw is under the dash. Use the star wheel on the firewall to adjust the free play