what pcm are you using?
if you have zzp's off shelf pcm the p0300 codes are disabled, (for cammed cars but on all their off shelf pcms)
I was having the exact problems (refer to Branson 2007) gas mileage went in the crapper, cut out, died, but would restart in a few minutes, but did get stranded on the highway, in a bad neighborhood, etc. I changed out the iac, map, fuel pump, fuel filter, and then finally put in a stock pcm... viola, code set! po305... misfire cyl 5... changed spark plugs, changed wires then finally coil pack... FIXED!
It's always a good thing to be around alot of people with the same car as you, I borrowed a coil pack from a friend to determine it was in fact the problem before I bought a new one.