I have a 1998 F150 XLT, 4.6 lt. V8. It has 127,000 miles, I bought it used about 8 months ago and it has been fine, I've driven all over on hunting trips with it. But whenever I either "punch" it or am going down the highway doing about 75 it starts to sputter, and almost quit. When this happens the check engine light starts flashing then goes away seconds later. Do I just need to tune it up or is this a more serious fix?

 

If you have no records of past maintenance , It's always a good move to do some routine maintenance so you know where you stand.
If you can, try to use Motorcraft spark plugs.
A new fuel filter is always a good move.
At that speed there is a high demand for fuel, and a restricted fuel filter could be part of the problem as well as a tired fuel pump.
Try to eliminate the routine maintenance items first, they are easier and cheaper to do.