I would suspect a lean misfire or a timing variation (getting too advanced).
BTW, I would take all the electric wires off the solenoids and tape them for diagnostic purposes. If you have two solenoids on the back, one is for decel cut-off. An intermittant application can cause severe bucking. The other is a starting solenoid (extra fuel). You don't want it involved after starting either.
Bosch Master, ASE Master, L1
33 years MB technician