stbkmac

I have a 1991 Hilux Surf in Okinawa, Japan.
VZN130, 6cyl, 3.0 ltr (gas)
transmission A343H with 98000 KM's.
Here is the problem-
once every four or five weeks the car would not shift from 2nd to 3rd (I thought it was a shift solenoid in the tranny, but no codes came up so it could mean the solenoid could slowly be going and is intermittent)
next the speedometer would just drop to zero but the tac would run just fine, the speedo is, I think cable driven so I thought " could be the shift sensor or maybe a type of short.
But now the Surf shifts without any problems and the speedo just drops off and works when it wants to without causing any shift problems.
could this problem be caused by a faulty shift sensor not a solenoid and where should I start.
the fluid is good (tranny-cherry red) I am at a loss and will check my speedo cable just to see if it might be rounded off at the tranny.
Last but not least could the ECU cause multiple problems like this????
need some help, other than shoot the car and get a new one