99' GMC Sierra...OBDII - Code PO300

So, the trucks misfiring really bad...seems to idle fine. i changed the fuel filter, plugs and wires and cap and rotor (recently), and cleaned the throttle body out. still, the same damn problem. im tryin to get it repaired before danger zone!!! any help would be appreciated...

Fuel Pump?

im going to try that today...i have a couple layin around...my buddy's suppose to bring home some sort of reader that works while you drive...we'll see. thanks for the help!!! any other takers??? 

STOP THROUGHING MONEY AT IT AND SPEND $100 BUCK TO TAKE IT TO A DEALER TO DIAGNOSE  

lol...every dealer ive takin it to wont touch it because its modified. its hard to find a mechanic thatll do work...

6 cylinder i'm quessing?

oh...i guess that coulda been some useful information...yesr, its a 4.3 v-6...has about 108,000 on it.

i have a 1996 chevy ext cab.i recently ran into the P0300 or random/multiple cylinder misfire.mine ended up being that the front 02 sensor plugs were on the wrongsides. This worked fine for a bit then messed everything up.cost me quite a bit for a new....-fuel pressure regulator-8 injectors-intake manifold gasket (which was precaution just because the truck has 130K miles and it was apart anyways.- i had to switch all of my 02 sensors.hope this helps a little.

thanks bro...i appreciate it...you had to replace all that stuff or is that the trouble shooting you did?

ok there are a few things to look at there....the connector going into the coil, the wires tend to break inside the covering.....pull on them with it pluged in if one breaks it could be part of the problem....also i have seen a few that the shaft of the distrubitor has worn the bushings....there will be a very slight amount of side to side play at the cap causing excessive clearance,i know it sounds weird that a very, and i do mean very small amount of side to side play would matter but i've seen it throw some realy good tech for a loop! this can cause a very random misfire on all cylinders....it would help to get a scanner on it and look at misfire data and see if its 1-2 cylinders or all cylinders at diff times...