What are some possible causes for hesitation when accelerating, bad gas mileage & loss of hp/tq?

Or rather, what are "hesitation when accelerating, bad gas mileage & loss of hp/tq" symptons of?

I bought the '97 Mark VIII Aug 2010 with 59k miles and did some maintenance jobs right away like new tires & full tune up etc. The vehicle has been running great up until a few days ago when I noticed the car idling a little rough when usually it sits so still & calm that I sometimes don't realize it's on. Apart from having the slight shakes, often when I step on the gas pedal(normal driving, not flooring it) whether from a stop or while moving, the car will pickup for 1 second and then it'll lag on accelerating a bit before picking back up. This hesitation has inevitably resulted in loss of HP & TQ on the Mark VIII. I have always used Premium 93 fuel and would average 19MPG while now I get something like 9MPG(per console cpu & various driving tests), an absurd decrease in mileage from an already gas hungry Ford V8 machine.

I already cleaned the MAF sensor, checked the plugs & COPs, checked for vacuum leaks, replaced the fuel filter & coolant temperature sensor. I have not tested the injectors individually nor the fuel pressure from the fuel pump. Until I do, what do think are causes for these symptons?