In experience with my car, I've had a similar shudder caused by two things.
1: Dying fuel pump. It would lose pressure under high demand when it got warm.
2: Dying tranny (TCC solenoid) and did it cold and warm.
Could also be a miss. Have your fuel pressure tested with the vac line off the FPR when the car is nice and hot to try and rule out #1.
I think #2 would give a code. Have it scanned or do the paper clip trick found in techinfo. The TCC code might not turn on the SES light, I can't remember.
The coupler could fail early, I suppose. I don't know what specifically would cause early failure, other than a weak coupler.
With the belt on, can you grab the belt and move it so any pulleys rotate some? If so, it could be the harmonic balancer dying. Look for cracks in the rubber on the crank pulley.