2003 Avalanche running rough

Filled up the truck the other day. 7 or 8 miles later check engine light comes on and starts flashing. p0300 random misfire. Truck has very little power and runs terribly rough. Get the truck back home. Thought maybe we had gotten some bad gas. Drained tank, replaced fuel filter, spark plugs... no help. With fresh gas, checked fuel pressure at the fuel rail. Turn key on and pressure goes up to 60psi, then drops down to about 52 or 53. Fuel pump? Fuel pressure with truck running bounces around alot. Is this normal? I'm about to pull out my hair. PLEASE HELP How much is it bouncing around? Give us High and low while engine is running. Also pull the small vacuum hose off the fuel pressure regulator which is the round deal on fuel rail, drivers side, if it has one, cant remember what year they canned them but take fuel pressure reading with that unhooked also with it running. Fuel pressure seems to bounce around 3 to 5 psi while running, down to maybe 52/53 and up to about 55/56. Very erradic. Will try reading with vacuum line off regulator. Really you need to find someone or take it to a shop that has a high end scan tool to look at the misfire history that is stored in the PCM. That way you can determine which cylinders are missing. That might give you more clues as to where to look.
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