My buick century has a acceleration problem between 30-50mph I've tried everything but can't figure it out,help

1. Install new ACDelco platinum spark plugs.
2. Change fuel filter.
3. Check for codes.

What year and engine do you have?

Well its my parents care so I'm not sure but it's a 99 and a v6, liter unknown we've tried the above suggestions including the catalaytic conv. And new coilpacks,ignitionmodule all fuel related scenarios. It just seems to be a hopeless and endless enigma. Thank you.

I would suggest at this time to have the computer checked as it may be malfunctioning and could be in a confused status at this point it's not a common problem but it is something I've seen happen