2001 s10 wcode 4.3 misfire on 3 cyl..spider or reg?

I have a 2001 4.3 vortec doing the exact same thing..runs great when first started..then 5-8 miles down road it runs rough under 2k basically..sets a check engine light & show to have misfire on three same bank cylinders..do you think this may be the fuel regulator or still possibly the spider?
W code 4.3 2001 model..replaced cap,rotor & fuel pump..
Thanks for any help..

Bad Air, or fuel problems can cause random misfire.

Air - filter, maf, iact, dirty throttle body, or unmetered air leaks on EGR, PCV, and intake.
Fuel - filter, pump , fuel regulator, injectors
Exhaust - Catalytic converters, mufflers, tail

I have seen a plugged cat choke half an engine out, causing random misfires.
Broken pieces of a cat can plug the down stream exhaust.

Did you read http://www.automotiveforums.com/vbulletin/showthread.php?t=324122
Bad intakes gaskets & seals will cause unmetered Air leaks,

I know this is a very very old thread but I have a 2001 4.3 vortec doing the exact same thing..runs great when first started..then 5-8 miles down road it runs rough under 2k basically..sets a check engine light & show to have misfire on three same bank cylinders..do you think this may be the fuel regulator or still possibly the spider?
W code 4.3 2001 model..replaced cap,rotor & fuel pump..

you need to check the fuel pressure..it must be 60-66 psi..key on engine running...with the key off the fuel pressure should hold steady and fall only a few psi in 10minutes...

if it falls off rapidly this may mean the fuel regulator/metering, injector , fuel pump....

to replace the regulator you must remove the upper intake manifold.

after that I would check for vacuum leaks PCV system etc...