Help....problem in Drive Line???

[sofia72]

I have a 1972 Fiat 124 Spyder with 210K miles, with a problem somewhere in the drive line (maybe) that I’ve been chasing for years.

Problem:
While driving around town in stop and go traffic at slow speed (2500-3000 rpms), in first or second gear the car will start jerking back and forth, especially if I go over a speed bump. Which means I have to constantly work the clutch to get out of this jerking mode, like I’m in the wrong gear. It feels like a lot of mechanical sloop between the engine and the rear wheels, like the rear axle gets out of drive sync with the engine torque i.e. to much backlash in the axle and/or diff, or worn-out components in the drive shaft. It doesn’t come out of gear, and I don’t have any problem shifting gears. However, I’ve checked and/or replaced just about everything in the drive line over the past several years in the following order:

a. Replaced drive shaft U-joints and rear wheel bearings and inspected donut coupling and center bearing.
b. Replaced complete differential pumpkin with a new one from Vick Auto, which changed the gearing to 3.90:1 from 4.30:1, I assumed the backlash was correctly setup.
c. Replaced the bushings in the trailing and link arms and inspected the engineer motor mounts.
d. Replaced all clutch parts including the pilot bearing and resurfaced the flywheel, which fixed a problem but not this one. Checked and double-checked the clutch cable adjustment.

Also, when in this mode, the tachometer doesn’t fluctuate, eliminating the engine as the problem. I also tested for clutch slippage by applying the brake while exhilarating in second gear.

I am considering replacing the transmission with a rebuilt one.
Any help would be greatly appreciated,
Thanks