[tombid]

I would check your spark plugs and spark plug wires and make sure they are gapped correctly, make spark, and that the wires aren't near grounding anywhere (such as the rubber insulator isn't melted and wires exposed). This could possibly cause what you are explaining.

 

[tombid]

Wrong spark plugs could cause the problem, but I am more thinking of a spark plug wire grounding out and not sparking hence you might be running on 3 cylinders instead of 4, could be brought up at full throttle as the time the spark is activated has a shorter wait period between.

 

[tombid]

The only thing that makes me question that it's a fuel problem is because he said the car consumes a large amount of fuel, my thought was the car was pumping a lot of fuel but maybe one of the other 2 parts to combustion was lacking. He said the car was getting around 16.8mpg which is definitely not normal even with a heavy tune