94gmcz71 looking for some help i have a 94 k1500 5.7 . it was a 1 owner ,it sat 4 years i towed it for $500.and got it running . I pulled the intake replaced the gaskets ,water leaking at the corner (fnt dr) at the head /intake..the basics ,new cap rotor,plugs,wires look good, fixed what vacuum leaks i could find.
breaks,tires battery, new 34 gal tank, fuel filter, fuel pump and sending unit all (spectra) ... starts great was running ok has always been a little sluggish and a little noise at 2300/2500 rpm but it has not gotten worse ( on acceleration )but running for 4 years not really any major problems until last week..when driving it on the the road / hwy everything is normal up to about 55-60 mph when it is in high and or shifting into o/d it feels like all of a sudden its running on 6-7 cylinders...power drop and vibrates.. ill pull it down into drive because it feels like it wont pull itself. ill turn aurond in the next drive way hesd bsck home and its running normal until o/d again..a couple of times after turning around it made it back with no problems. and when i get back it will idel great ,rev good , stop and start np..and make it back out to the hwy..but repeat once we get going.. no back fire ..it seams like fuel shortage..? or timing advance ? or lack off.

any ideas

 

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