95 Blazer vortec engine hesitation
 

My 95 blazer which I've had for about 8 months now. There has been many parts replaced and fixed. It has had a small tune up spark plugs and wires, spider injectors have been cleaned, new fuel lines, new fuel pump and I just gave it its first oil change which was black as night with some new high mileage stuff. But for some reason at 40-50 and 70-80 mph the engine feels like it is having a slight hesitation I would say. The check engine is on but no one can pull the code because its a OBI/OBII mix. I was wondering if anyone else could help me please and thank you very much.

Update
 

We found a vacuum hose that wasn't plug in at all and now the check engine light comes on and off but the engine still feels like its struggling at certain speeds