Problem after engine changeout, 99 Neon

I completed the change a couple days ago. Vehicle starts, runs fine. After
15-20 minutes on the road the speedometer becomes erratic, fluctuating
rapidly up and down, eventually after about a minute, settles to zero, where
it remains until the vehicle is stopped and allowed to sit for a while. The
auto shifting becomes irregular after the speedometer with higher than
normal revs being required when it does finally change.
A 30 minute cooldown period allows the vehicle to perform correctly for
about another 15-20 minutes at which time the erratic behavior happens
again.
I have seen posts indicating the eeprom might need a reflash.
Does anyone have any ideas about this problem?
TIA
CenturyTek

 

There is definately something with your computer/sensors. I know that
until the engine reaches running temp, there are verious sensors that
are set to open circuit. I do not know the whole list of them other
then the O2 sensor, which would not cause that issue. The engine you
put into the car, was it the same as that that came out? If the
transmission and engine type are not the same as what the ECU was
programmed for, this would be your issue.

 

Sounds like a speed sensor problem, or the connections to it. Did the
transmission get changed with the engine or is it the original? When I
had my transmission rebuilt almost two years ago, something happened
to the sensor as a result of the transmission removal/installation.
The speedometer was erratic, or inoperative. I probably should have
gone back to the original shop to have them make it good, but I did it
myself in the interest of time.

-Kirk Matheson