History-Bought vehicle run down. Fixed it so it runs well. Then had a random misfire. Tuned it up with plugs, wires, rotary cap, and button. Ran great. 2 months later it happened again. Took it to a mechanic...he suggested spider injectors. I replaced them. Same issue. Was at the end of my rope looking at the engine running at night when I saw a spark.  Brand new wires were arching. Replaced them. Runs great. A few months later... misfire again...noticed one morning fuel was dripping out of the evaporator vent valve. Replaced. Ran well for a couple of days but the misfire returned. Filled up with gas one day and noticed fuel leaking from the vent valve again. I did a vacuum test on vacuum lines. Had -17. seemed to be okay for my elevation. I am wondering it is the plugs again...but I just can't imagine its another brand-new set. One thing I should note...I put in Flex fuel and am curious if that could have damaged the plugs? Any thoughts Scott? code, p0171,p0174,p0455,p0507