97 riviera speedometer jumps car sputters

Is it ecu? Flash what?

I suspect it's the ECM ( engine computer).

I was on my 3rd submerge, my friend. Finally, someone throws me a line. That's what I initially guessed, but what to do? I read that the replacement has to be flashed? How hard will it be, to say, buy one from rockauto, then put into service? Any more rope?

First, have a scan performed to be sure its the ECM. If it is, one may locate a replacement in a salvage yard or at an auto parts' store. Be sure and get a few days warranty if possible, if getting from a salvage yard. The ECM is located behind the vehicle's glove compartment.