The only electronic control is the kick down solenoid. It is at the passenger side rear of the trans and can be disconnected by unplugging the single wire. I would do this and see if it is eroneously downshifting electrically (that would stop it).
If not governor pressure should be tested. If the trans has over 150k and has intyernal leaks its best to disassemble clean, reseal and replace all the wear frictions and thrust washers. sort of what they call a rebuild.
Bosch Master, ASE Master, L1
33 years MB technician