The ABS system on the W126 is just that.....Anti-Lock Brakes. If you hit the brakes hard enough to lock the wheels, it pulses the brakes to keep the wheels from locking and sending you into a skid.

Self-applying brakes on Mercedes didn't come around for over a decade after your car was built, and only to prevent a crash! The brakes should never apply themselves on a W126.

Since the jerking happens when you accelerate, let off the gas, then accelerate again, it sounds like you have a transmission sticking in a gear or stumbling during a gear change. If you're not technically inclined, it would be well worth having it checked by a reputable shop before you do any permanent damage to it.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]