My car is missing on one cylinder?

I have a 2000 deawoo, 2.0. the thermostat went bad and overheated. I replaced the thermostat but now it misses on #4 cilinder. I changed spark plugs and plug wires. I put additive in the oil, and injecter cleaner in gas. After 3 days # 4 started firing. A day later it stopped and hasn't fired since.