Hey guys, first post here and in need of some serious help. Ive had a 98 Blazer LT with 4.3L vortec for a couple of years now and its ran great for me. Just recently upon driving out of state for school, it started acting up. Ive had it serviced 4 times at 2 different shops with no final solution to the problem. The idle is rough at times, the truck occassionally stutters from a stop, the main issue occurs at highway speeds where it thumps with minor stalls, then will stall completely and or die leaving me coasting to a stop on the side of the road. It has also died at idle. When these issues occur it often misses and runs pretty rough. The thing that has me confused is that it does not happen all the time, leading me to think it is an electrical issue with a sensor causing false readings. The truck is not currently displaying ANY related codes. Fuel economy has dropped to an average of 15.5mpg highway.
This is what i have done so far in the past couple months to try and fix the issue
-replaced IAC
-cleaned EGR+new screen
-checked for vacuum leaks
-new fuel pump
-new distributor
-new distributor cap
-removed catalytic converter
-new muffler for better exhaust flow
Truck is currently in the shop right now leaving me without a vehicle out of state. Really starting to **** me off! If anyone has any tips on what the issue could be, please let me know. I'm just about at the end with this truck.

 

Have you done a fuel pressure leak down test? What are your fuel pressure readings with the key on, engine off?