Truck runs good while idle

And when get going it starts stalling and the breather top pops up like it is keeping from back firing

changed the spark pklug wires in April of 2013, but noticed the truck would slow down on the freeway when get to about 50mph, changed the wire that was damaged to the spark plug, but not all the wires then it was ok for a day then the same problem.

So I changed the fuel filter thnking it was the problem truck ran fine till later that night on the freeway. Now changed the wire on the distributor to the core and everthying was good had power, did good on the freeway 70mph not issues, until later that night the same problem came back