Hi
it appears as if the LFO itself is faulty.

Did you try Split Sounds For Single sounds only the primary LFO circuit is
active. For Split/Double sounds two separate LFO circuits are effective. If
the primary LFO is defective on the first programme, you should be able to
have LFO modulation in the second programme of the Split.
If both LFOs are failing, the problem is probably with the control panel
itself, or the ROM-program is dying (EPROMs were meant to live 20 year at
max) .

Cheers
Mondo