[Beav]

The torque convertor has nothing to do with the tach. However the crank sensor probably provides the signal and the engine speed is used to determine certain parameters of the transmission's operation. But since you haven't mentioned the check engine light flashing I'd suspect the tach is sticking a bit. If you're the handy type I'd suggest pulling it out and checking it for debris, cleaning it up and maybe a drop of machine oil on its bearings.