Auto Trans downshifting all the time on my 92 Maxima SE

[computermech]

I've got a 1992 Maxima SE with 115 Km on it. I am the 2nd owner. At 89,000 KM for some reason the 1st owner got his transmission replaced with a "new" transmission by Nissan dealer (not rebuilt !). He showed me the reciept. So, for me the tranny is almost new.

Problem: is that as I am driving, the AUTO transmission jumps into a lower gear and the Power Light on the transmission switch starts flashing. Then I slow down and the problem dissapears after a few seconds. This happens all the time intermittenlty. This happens as I
accelerate in slow or high speeds.

I have run the transmission diagnostics and found nothing wrong
as all intervals are the same on the P light flashing.

Since this is an almost new transmission, could it be the mount or
may be "Throttle Position Sensor" or "Torque Converter ".

Thanks in advance for any advise. (I have searched the forums).