1996 Sierra runs rough after tune up

1996 GMC Sierra SL V6. Owned since new. 100,000 miles, never tuned up. I decided to get a tune up in hopes of increasing mpg, and it was overdue really. New plugs, wires, cap, rotor. Got truck back and it runs rough, knocking, skipping, hesitating, bogging down when trying to accelerate, rough idle. Went from 15.5 mpg, down to about 14 mpg. They said they put in all AC Delco parts. Brought it back to them twice now and they cannot figure out the problem. It ran PERFECTLY before I send it in for a tune up. No problems whatsoever. Very puzzling. Any ideas?

Maybe they dropped and/or cracked a spark plug, or crossed a plug wire? Either way, if they caused it they need to retrace their steps and fix it.

They just called and said something about the air intake system was not connected or something along that line... said it seems to run fine now... could that have been the problem? How could they have messed that up doing a tune up? Do they have to disconnect any part of that system to do a simple tune up?