MY first suspicious would be too little fluid to act on the solenoid, but it sounds like an electrical problem.

Your 4th gear lockout may be part of the problem allowing you to slip into OD in 2nd gear. This is a serious potential problem since the OD is ruined if it is run in reverse. A common and very serious warning.

You intermittent operation may be the relay or the wiring to/from it. If something is grounding or arcing past the normal circuit weird things can happen.
I forget if you have anything really weird in that circuit in a 123GT - they are well outside my experience...

I would suspect the wiring first, and gearbox innards second.

There are some other serious potential problems, but check the easy stuff first. The OD indicator light is a pretty good check of relay function...

Just my two cents.