1994 f150 i6 strange engine problem

Alright so im new to this forum, but i have a very urgent problem. My truck sometimes acts like it wants to go dead while at idle.. yet it also does so when driving. Ill be driving down the road at constant speed of about 45 or 50 then its starts cuttin out acting like its gonna go dead and if i do nothing and retain my foot on the pedal it gets worse, if i accelerate past it its ok but sometimes makes it worse. When it cuts out, it feels like i lose all power to the engine… I have no idea what to do, shes an old truck and stuff has started needing replacing so i have replaced alternator but am working on other things just dont know which direction to go. any info would be great thanks…