shudder at 55-65 mph

my 1981 vette has a shudder at 55-65 mph and is more noticable during accelerations. above 65 it goes away and below 55 it's not noticable.

while coasting down hills, if i throw the car in neutral, it rides smooth as silk, so that's telling me it's not in the drive train. all u-joints are new, tries professionally balanced, all new rear bushings and alignments done.

should i now suspect the transmission or torque converter? stall testing does not produce any vibrations, so i'm inclined to believe the torque converter is acting up when warm or possible transmission plates are slipping (?)

this is driving me nuts since i like things to run perfectly..lol

suggestions anyone??

suggestions anyone??[/QUOTE]

Well,, drive under 55 and over 65..... :iceslolan

Seriously, Check you motor mounts. The rubber may be shot.

The only other item that comes to mind is bad u-joints at the half-shafts.

Do you feel the vibration in the whole frame (thru the steering wheel?)
or only front or aft?

car has all new u-joints. i have checked engine and tranny mounts.

when car is sitting still and i bring up the rpm's...no shudder or vibration.

when i put it in gear, hold the brakes and bring up the rpm's to almost stall...no shudder at all.

when coasting down hill and in neutral...nothing.

this only happens when it's been warmed up and driving at hi-way speeds (55-65 mph) and when i accelerate it get's worse until i go over maybe 70...then it's gone.

based on all that, i'm inclined to believe it's the torque converter TCC solenoid locking in and out, but, that's just a guess at this point.

I have that problem with my 1992 I think it is the torque converter when accelerating after the lock up converter has kicked in, I get a shutter that I can make come and go at will. I have not been too upset with it to have it fixed yet because of the cost.