Why do M-7s jerk back and forth?

When riding M-7s on the Harlem Line today, and from my recollection of M-7s on the LIRR Main Line, I noticed that rather than just staying at a constant speed, the cars seem to jerk back and forth. Although I'm sure it's not the case, if an M-7 were a car, it'd be as though the brakes were being tapped and then the accelerator were being stomped on.

Is this a track problem, a truck problem or a motor problem?

Thanks.

I have also noticed that,unlike m1 or m3 you cannot simply coast thru gaps in the 3rd rail. there seems to be some type of automatic braking when you lose 3rd rail power. makes for not a smooth ride . This problem along with the slackless coupler c-3 fleet on lirr leads me to think, A-mta doesnt care about a smooth ride or B-people designing and ordering this equipment are not railroaders. and it takes a lot of job satifaction away from me when i am overridden by a computer.EX- I know when to lay sand down but the computer knows after its too late

Although I have never RUN them (or even seen the cab of one lol), speaking from experience with running Arrow MU's, perhaps they have a 2 or 3 position controller similar to Arrows, Silverliners and such where power is only applied when necessary and not gradually like a typical "notched" throttle. It's either all, a little, or coasting. And I believe when these things go into coast mode there is a little regen power created for HEP.....probably the light braking sensation you feel. I'll defer this to the guys who run them, but that's my quick guess.

The M-7a's have a linear controller no steps/notches other than where cab signal suppression and idle is.
The jerking occurs when certain cars do into braking mode when third rails arc to much or when cars go trough gaps in third rail.
There is nothing engineer can do about it, just a little quirk in the AC propulsion system that engineering department never fixed.

I do not notice the lurching/jerking too much, its the squeaks and bangs at the B ends of the cars that get my attention. Way too loud and with the automated announcements every stop...your ears never get a break. I'd rather ride the M-3a, smoother and quieter ride.