F150 300 Engine Has Miss

I Have 1989 F150 200,000 Miles That Has A Miss While Traveling Down The Road. It Seems To Idle Ok Most Of The Time. When It Starts To Miss If You Give It Gas It Stops The Miss Until You Settle Out To Whatever Speed You're Traveling. Some Times It Will Act Up More Than Others. I Have Had The Fuel Filter Changed But I Can't Determine Whether It's Fuel Related Or An Electrical Problem. I Just Had Some Problems Fixed On The Fuel Pump Not Geting Enough Voltage. But The Miss Has Been A Problem That Has Been Getting Progressivly Getting Worse Over The Last Year Even Before The Other Repairs.also Switching Tanks Makes No Difference. Any Ideas??

how many miles is on the engine?

I don't know for sure. I put around 50,000 on it since I've had it. It appears to be original.

Have you got into an accedents, no matter how small the collision if the ECU got bumped the right way it could ahve messed up the timing or the mapping.

Does it only miss at high rpm? Have you checked to make sure your getting all of plugs to fire in the correct sequece?