Sounds like your coil is going bad. I posted about this awhile back,
http://www.automotiveforums.com/vbul...highlight=jgr7
Any ways what happens is there is a resistor inside the coil that when it gets hot affects pulses going to the tach which causes it to bounce. Also it will feel like the engine is missing , but the transmission is really going in and out of converter lock up. It feels like a miss in the engine. Change the coil and it will be good to go.
Jeff