i have a 1990 that all the same problems u are having and its sounds like alot of other people . So like u said it starting running ruff feeling like it was going to die then it did staring dying stalling out at random so i staring the replace alot of things first the fuel filter then the cadi converter then i starting getting into the distirbutor anyhow it lasted months then fnally said f it and changed the whole distirbutor and it ran great for a week and then stalled on and the computer was telling me the whole time it was the distirbutor so i changed the computer and its ran great but my water pump just went out like a week ago but i've already replaced it and that was a pain in the butt
My 90 deville did the very same thing yours is doing, drove me crazy for months, replaced distributor, plugs, wires, fuel pump, filters, vac lines, cat convertor, to no avail. Then I was almost ready to give up, I tested the fuel injectors, egads ... only four were working. I replaced them with the bosch design III. This car will now run at 34mpg on the highway ! Well worth the investment and fixed the miss and roughness. If you do this, I suggest to replace the fuel regulator as well, it will help since you have the fuel rail off. You will have to loosen the ps pump and slide it back to get the last injector out. Don't forget to relieve the fuel rail of the gas and don't turn the key until you are completely done or take the fuel relay out.
Use Lucas Injector Cleaner or Sea Foam Injector Cleaner use eather one about 4 oz throu large vacume line from brake booster to yo motor with the motor runing the vacume will suck it throu the intake . Make Sure The Tail Pipe An The Hole Car Is Not Around People A Large Amount Of Black Smoke Will Come Out Of Yo Tail Pipe . an put about 12 to 16 0z in a tank of gas every 3 months.