It sounds like your having a transmission problem.
There is a lot of things that could cause this in a transmission, including a bad siloniood or it could be a clogged filter.
My '89 new yorker weighed 3280 lbs when I sold it for scrap this morning.
It is behind the battery.
Look at getting a new transmission
Yes.
the fuel pump is located in the gas tank
It is under the housing where the upper radiator hose hooks to the engine.
maybe an ECU problem
through the tranny dipstick tube with a tranny funnel.
1-2-3-4-5-6
The engine computer is the voltage regulator. It is behind the battery.
does it happen a few seconds after you start the car? on my 1988 new yorker,it was the hydrolic pump that is on the right side of the trunk. i just unpluged it.
Once the computer tells the transmission to downshift the car should in theory downshift just as quickly as it upshifts.