Could it really be the original rotor

[SoDak]

Hey guys, I drive a 90 sx and about a month ago the gas milage went south for the winter. Well I was replacing a bearing one day and on the restart i noticed the ingine was missing. At high RPM it drives fine but at idle it sounds like it's on 3 cylinders. So I replaced the plugs and wires. Still nothing. So I took off the rotor cap and there was a lot of build up. I cleaned them off and now it runs even worse. I can not adjust the rotor any further. I have a rotor and cap coming tommorow. Do you think this will solve the problem? Is there more information you need?

220,000 miles and running great up until now. If the timing chain would have skipped a cog the car wouldn't even start right?

Thanks