I have an 86 tdtII doing very similar behavior. Starts up at ~60 psi and goes down to about 30 whlie drivning in town. It disconcertingly heads down to 0 when on the highway, however the idiot light stays off. Sometimes it starts reading again when back in town after a highway trip. I haven't been doing any highway miles because of this.
I looked at it today, and the sender has a small external 'condenser' held on w/ a pipe clamp & *****.
I also have an 88 2.6 trooper and a spare oil sending unit for it - anyone know if the sending units are interchangable between the two? I can't see why not, however the setup for the gas trooper never included the condenser.

Thanks for any help