Bad gas, fouled plug(s), engine out of time, vacuum line leak or clogged.
Was it missing and sputtering before you changed them? If no, then you could have got the firing order mixed on the distributor. Maybe if you didn't replace the wires also one became more damaged while removing it. If yes, maybe its your fuel, or injectors, and you need to add a bottle of cleaner to your gas tank.
You need to check compression on #6.
Sputtering is almost like a person spitting. The car runs but it doesn't have full power, could actually sound like someone spitting. Missing is when there is a consistent and constant slight jerking or vibration. Unless it is really bad it is hard to feel.
Time for a tune up.
A missing lug nut can be quite dangerous for anyone who is driving. This means that the wheel is not as tight as it should be and it can come off especially when one is driving at high speeds.
could be a leaking vacuum hose or missing vacuum system cap
There is a shield that prevents water from getting on the PS belt. It is missing or damaged. Also check to see if the belt is properly tensioned.
Missing information: How fast are you driving?
Depends what your country is.Re-Ask with the missing information.
Electronic transponder microchip in key is bad or missing.
You are driving with an invalid registration. That means you can get a ticket.
I had a '98 Bonnaville do that, among other things, run rough, hard start, missing & sputtering. A year or so after I got rid of the car(MAN, I loved that car!) i got a recall notice saying that the FUEL PUMP RELAY was being recalled. I don't know if that's your problem but it would be a good place to start.