I've got a 305 with a T-5 trans. All the gears shift smooth and have no problems while driving. but today I was driving down the road and I put it in 5th and the gear shifter started shaking bad. I put it back in 4th and drove for a short distance, then put it back in 5th, it shook worse this time, felt like the trans was gonna jump through the floor. I stopped and went through all the gears again. everything was fine until 5th again...seems like a timing issue...what do you guys think?

 

That happened w/ 5th and 6th gear in my 94 formula. Turns out I had a burnt plug wire... don't know if that would help you or not...

 

It gives me somewhere to start at the least. thanks.