if its a vortec , it may be developing a problem in the poppet (injectors)
you could run some seafoam or chevron with techron through it to see if that helps.
Just dump some in a 1/4 tank ( I like to dose it heavy) and drive till almost empty, then fill up and dump another can in it. Drive it a while, see if it clears up after a while , if not maybe it will set a code and help aim your diagnostics in the right direction.
Good move doing a tune up first. Nothing I hate worse than trying to diagnose a driveability problem with all the regualer maintenance items in the questionable area. Eliminating them is must for any good diagnosis.
Inspecting and / or replacement is needed.