1993 Nissan Altima bad spark plug or wire

Fire was breaking in a spark plug or wire after the vehicle engine warned-up, only in the driving position at a stop sign or traffic light. So, I had it checked at a license auto repair shop. They checked it out and found out that the #3 spark plug was wet with oil. They informed that the valve cover gasket had to be replaced. When doing so, thay also used a intake and combustion chamber cleaning kit that came with the gasket. I went to pick up the car and it had a rough idle. The engine was shaking real bad. So, the mechanic inform me that I had to burn off the chemicals that was applied in the engine. It's been about a week now and the engine still has a rough idle to it and it shake now going into reverse while backing out of my driveway.