1998 Nissan Pathfinder Running rough and lacks power.

Engine Performance problem
1998 Nissan Pathfinder 6 cyl Four Wheel Drive Manual 185K miles

The check engine light was flashing and the truck was running very rough and lacked power. It will do this for 5 minutes and then all of the sudden it will clear up and the check engine light goes off the the truck runs fine. It has done this many times over the last week. I pulled the codes and I got a cylinder 6 misfire and a Knock sensor code. Any ideas where I should start. The cylinder 6 spark plug is so hard to get to I didn't dare take it out for fear I couldn't get the new back in.