Miss firing

Diagnosed check engine light on 99' subaru outback impreza sport with 130,000 miles as a misfire of cylinders 1&2. I changed plugs, plug wires and coil pack. After coil pack seemed to run better, but light now flashes several times when over 3200 rpms. Also, the head gasket was replaced 3 months ago and the front of the motor where the belts are at seems to really be shaking now. What else could cause a misfire of the cylinders? Im concerned this is another $1000 if I don't repair it soon but I don't want to get ripped off- thanks