i recently took my blazer to a local shop on basis of random misfiring.
they replaced the EGR gasket, however it continued to missfire.
the garage said i needed anew intake gasket and rear main seal, they wanted $1200.
oh man, i'd rather do it myself but i was hoping someone could help me determine whether or not the engine will start or not after i replace the intakegaskets. and any clue on the misfiring?

 

Are you leaking oil? If so, maybe try tightening up the oil pan bolts first to see if that helps. If you remove the torque converter sheild and see oil on the front of the flex plate, then the rear main is leaking. You have to determine if the leak is enough for you to mess with as it requires the removal of either the engine or the transmission to replace.

Oh the random misfire, move around the intake with an unlit propane torch and the engine running. Listen for increased engine RPM as you trace around all gasket mating surfaces with the torch tip. Follow all vacuum lines. This will allow you to pin point if there is a vacuum leak.

Other preventative maintenance things that can be done are to actually clean the EGR valve, not just replace the gasket. Next, clean the MAF sensor with some sensor safe carbon cleaner (or MAF specific cleaner); run a search and you should find some pretty detailed instructions on this. You may also want to clean the throttle body.

There is another thread with something like P0300... something... Anyway, that thread goes through pretty much all of the causes of a random multiple misfire.