The fore and aft rhumba is caused by the accumulation of play in the various components in your driveline.

First the engine fires and pushes the car, then the engine slows and the accelerating car pushes the engine. Then the cycle repeats itself.

With the engine off, the emergency brake off, and the transmission in first gear, try to rock the car back and forth. Any movement is due to slack in the clutch, the three universal joints, transmission gears and splines, and rear end gear lash, etc.

As your car has worn, all these slight slacks add up to the jerkiness.

Use a little more throttle when you take off and stay well above idle as you drive in first gear.

I had a 1960 SAAB that had free wheeling - it would coast whenever the car moved faster than the engine. If you let the gas off when you went down a hill on the highway, the engine would drop to idle until you stomped on the gas in order to accelerate again.

One trick that this car (with its tiny, three cylinder, 2-stroke engine) would
do is that with a full load, you could let out the clutch in top gear and pull away from a dead stop and the car would not stall. You could feel each power stroke push the car, then the car would coast a bit. The car would go bump, bump, bump-bump, bump, bump-bump untill it picked up speed. Some times it would take a mile to get up to 20 miles per hour, but it would not stall or rhumba!!!

An automatic transmission, like my SAAB's free wheeling, will not rhumba.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 415,000 miles put on 9 bricks