1991 Buick Century Running rough and stalling

First check all vacuum lines to make sure you don't have a leak.

If you eliminate this as a potential cause, then check spark next. (Starting with the base timing. Make sure it's set at 6-degrees BTC. If not, set it.)

With the engine running, pull one spark plug wire at a time. If the engine drops down in r.p.m.s, and/or runs rougher, then you know that cylinder was firing correctly.

If the r.p.m.s stay the same with no noticable change in roughness, then you know that cylinder was NOT firing.

Now you'll have to find out why. It could be inside the distributor cap and/or rotor.

If you eliminate the ignition system, then you check fuel next. The fuel pump, fuel filter and one or more injectors could be bad.

If the fuel system isn't bad, then you check the sensors, one at a time. (Crankshaft position, MAF, MAP)

Don't just take parts off and replace them. Doing this will NOT work, and will only be expensive. If you run into a suspected part, remove it and take it to AutoZone or any major parts store and ask them to check it. They'll usually do this for free as long as they're equipped to do so. If they aren't, but you're trying to save a little money, take it to a mechanic and ask them to check it.