So I took my Jeep to a highly rated Jeep / Offroad shop to have the oil cooler replaced. I picked it up and everything seemed fine but later that day I got a check engine light (P0306 misfire). I took my jeep back to them and they said it was probably one of the seals and they would take a look. Two weeks later I got my jeep back. Again seemed fine but shortly I got the same error code. Jeep seems to run fine I never notice the misfires and sometime I've driven 100+ miles, after clearing the code, before it comes back. I removed the upper intake manifold to check the seals and they all appeared OK. I also swapped the spark-plug and coil pack from cylinder 6 to cylinder 5 to see if the error code followed either of those, but I'm still getting P0306. Also it doesn't seem to be overheating or show any other symptoms.

I'm now also noticing engine coolant levels are going down. I don't see any evidence of coolant on the oil dipstick and I don't see any coolant on the garage floor?

I'm not a very experienced working on vehicles. What should I check next.