computer problem in 99gt's?

Hello my buddy has a 99gt, im asking for him cause i dont feel like signing off, well his car isnt the fastest car around, he has most of the basic bolt ons, 3.55 gears, cai, t/b, o/rx, flowmasters, mgw shifter, 315's in the rear, and hes running 14.5's, and he gets ****ty gas mileage. We heard there was a problem with the computers, is this true? could this be why his times are so bad and gas mileage? is there any way to fix it? or does he just suck at driving cause the 60' time is 2.0 and the mph is up there but et's are just ****ty, thanks