i have a 84 s10 blazer 4x4 w/2.8 stroked .30 with a 5/10 cam and flat top pistons, hedmen headers and a edelbrock higrise 4 barrel intake with a 390 cfm holley. when i first bought the truck it ran great. although i have tweaked it since. i got the secondaries working(not enough pedal travel). now i was going to a off road trip and on the way i noticed it started stumpling with driving on the interstate. i pulled over took a quick look nothing looked wrong so i got back in and drove on pedaling it. when i arrived i took another look because it was still doing it. i checked and found that the truck had a plug on the pcv part of the carb. it was missing so me and a buddy rode up the rode to the autoparts store. got a new plug put it on and it ran fine so we went 4 wheeling. when i left after back it started again. well i also noticed i blew the rear main out after a shadetree mech (friend of my dad) redid the rearmain and screwed the timing up i towwed it home timed it and did tune up including plugs, wires, cap and rotor. got it running and test drove it and it was still backfiring so i retarded the timing down and it quit. well a month ago i changed the plugs and it stared again so i retarded it and still doing it i have since rebulit the carb, put new intake gasket on, new vac lines and still no go. i change the plugs to stock ac delco and it got better. still doing it a bit so i went down in heat range and changed them again. i have also a vac leak from my brake booster i am going to swap this out this weekend if i get time. any one have some ideas on what may be the problem