Mine had this exact problem. I found that it was as simple as timing, and the two carburetor adjustments (idle speed & fuel/air).
Before assuming your torque converter is screwed up, make sure you find the correct specs and take the half hour to time it right.
Idle Speed: Roughly 550 when in drive with brake on. (Ends up being like between 700-850)
Fuel/Air: Turn screw in all the way towards the carburetor (DO NOT TIGHTEN, carb can break)
back out 3 turns. Start motor. TUrn in the screw slightly to gain best engine smoothness as possible (usually a 1/4 turn in after the 3 turns out).
The timing should be 4 degrees BTDC.
I've seen a lot of people on here say different things about timing that just sounds... well, mroe trouble then it is.
Just go by those specs (or look them up in "The Book" yourself and you'll see that they're right). Either way, timing is the FIRST THING to try for something like this.
