The ol' girl gave me a little trouble this weekend, I started having a missfire at part throttle. I would have suspected bad gas or carb problems but the tach was jumping around during the missfire. Roll on the throttle and put it under a load and no misses and the tach would stabilize. Checked all the grounds and connections, any sugestions before I start replacing the ignition?

 

If I remember correctly this is a classic sign of a bad coil. Search the site for threads with ignition coil in them and you will likely find it.