I have a thought for any of you with this problem. I've never worked on a 2120, so I'm not familiar with its control system, but I have worked on many brands an styles of electrically controlled transmissions and shuttles. Try this: attach a volt meter to the fwd solenoid wire at the post on the coil. Sturdy enough connection so you can operate the tractor for a while and not lose connection. A meter won't affect the operation or confuse any controllers involved, but it will tell you what the system voltage is to the coil during normal operation. It will also tell you what that voltage is when your failure occurs, assuming you can get it to fail while still hooked up. When failure occurs, the volt meter will tell you if your problem is lack of signal(reduced or 0 voltage), or valve/clutch pack related(consistant voltage and no clutch engaged). If it's signal related and you're quick enough, you may be able to back track with the meter to determine where the signal disappears.
