I have a 2006 Suzuki Forenza with roughly 110,000 miles on it. About 6 weeks ago the check engine light came on and I pulled the P0300 (Random Misfire Detected) code. I went to check the plugs and noticed they were covered in oil, so I changed the valve gasket as well as put NGK plugs and new wires back in it. The misfiring initially went away but 2 weeks later it came back. This time the check engine light started flashing and there was a brief loss of power that lasted for about 20 seconds before the engine light stopped flashing and the power came back. So I checked the plugs again and they are not covered in oil, they look just fine. This time I got the following codes P0300, P0131 (Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)) and P0420 Catalyst System Efficiency Below Threshold (Bank 1)). I cleared the codes wanting to make sure that those codes were valid. The check engine light hasn't come back on since. I took the car down to Midas to have the run the full system diagnostic on it and they can't pull anything. They drove the car and felt the misfire like I did but the computer shows nothing. They checked the plugs, wires and coils and all checked out fine. I suggested looking at the O2 sensor and they said both of them check out just fine. They thought I might have gotten some bad gas as the pulled a sample and it was a pretty nasty brown color. I took it down and put premium in it but it's still doing it. At this point Midas has no clue and I am stumped. Any suggestions? Oh also the Timing belt, serpentine belt, water pump, egr valve have all been replaced in the last 8 months.