Update on Do Not Shift light

[trautoranch]

ok had a trans tech lined up to come out this morning (99mi one way) He
called us this morning and ask a lot of questions. I memtioed that the
tachometer kept jumping around from 6 to 1600 rpms. However the engine
sounds steady. Now the tech says it is giving a false reading to the
transmission not allowing us to shift.(Do Not Shift light still on).
The tachometer does remain steady on high idle however.

Any Ideas?

Rick & Terri 1986 8V92
Patillo, TX