I have a chevy S10 2000, with a V6 engine and it's four wheel drive. It sat for awhile and I replaced the fuel pump and filter and it has ran great. But whenever I go on freeways or drive in the down town area with lots of stops and goes my truck will stall. It only happens when it's hotter outside but I check the temp gage and it's always under half. I have no idea what is wrong with it.

SOURCE: 2000 chevy silverado 1500 4.3 eng

Chevy Trucks, Blazers, S10 pickup and the like will vapor lock ecspecially the 2000 models. Do not tighten the gas cap past 1 click. There is a tube that runs from the top of the tank down and this gets hot and collapses or burns in half. We just bought a reinforced rubber tube cut it to lengh and use it instead of part and it's worked grat for 2 years so far.

SOURCE: 1999 Jeep Grand Cherokee stalling and not starting

Since you've replaced many of the usual suspects (fuel pump etc.) you might want to look into testing or simply replacing the crankshaft position sensor or the camshaft position sensor. If you intermittently lose one the these signals it would cause the problem you describe.

SOURCE: 98 blazer stalls

It could be your fuel pressure regulator, check this link for details:

http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c1528022b9c4

That's what it looks like.

SOURCE: 97 Chevy Blazer: 4WD 4.3L 6cyl Runs great all

have fuel pressure tested has to be over 80psi mine was doing same thing first fuel pump had installed went bad after 3 months guy didnt change filter in tank.mechanic said never let go past quarter tank!

SOURCE: 1994 GMC JImmy 4.3 V6 running very roughly

fuel pump sock or try the egr valve to see if it is stuck open all the time. can't remember if that one has the digital one and a vachumm type as well, next would be the evaperative canister purge valve.