2000 Backfiring thru intake

Hey guys I bought a 2000 Blazer 2 dr. 4x4 4.3 auto a couple of mos ago and its got me stumped. It starts and runs fine unless i give it more than 1/3 throttle, then it sputters and or backfires thru the intake and loses power. This will happen on side roads or on the freeway. I have replaced the fuel filter, plugs, wires, coil, dist cap, rotor and cleaned the MAF. Any other ideas? Fuel pump? Thanks.

from having an elcamino with a 454 in it and having it pop thru the carb, it turned out the cam is shot on 2 cylinders the lobes are round instead of eggshaped,,,, So in turn theres backfiring thru the carb and the exhaust...  How many miles are on it?   What # owner of the veh are you and do you know any of the background of the truck?  The truck might have been beatin and abused and thats why it was sold in the first place..

The woman i bought it off of bought it new (yuppie type) so i dont think she ran it hard. 98k mi.

what kind of gas are you using and have you put in any kind of injector cleaner?? I use Marvel mystery oil in my blazer havent had any probs yet.. I add it to the gas each fill up..

Just 87 regular, various brands. I do add injector cleaner at every oil change.

I GOT IT! Thanks to garry2000blazer in the DIY- HOW TO forum, I disassembled it all the way, cleaned it correctly, and it runs like new. Thanks again guys!

disassembled what item??? 

Probably the EGR valve.

Sorry ya'all, I ment the MAF as seen on garry2000blazer post in DIY- How to, it worked out perfectly for me. The truck runs great!