96 Blazer

I have recently acquired a 1996 Blazer ,all is great with it , but at about 50 miles an hour ( 2000 RPM) the engine seems to lope and/or hesitate,,,,,the tach goes crazy ,,,almost seems like the engine is fuel starved,,,,replaced fuel filter ....replaced distributor and all electrical components in the ignition system....replaced the computer....a tune up was done just prior to my taking posession of it...the Catalytic converter has been replaced
( 11/ 03)....almost everything that we can think of has been replaced ?

Any thoughts ???
Thanks in advance

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