Fuel injector with no voltage

I have a 2003 Dodge Durango 4.7 2WD SLT. I noticed that it has no power on acceleration. I bought it used and got a diagnostic on the CEL. It said I had misfires on several cylinders. I got new coils and new plugs for all eight cylinders. I ran fine for awhile. Then, it began to run sluggish again. I checked the coil harness with a noid test light and everything was good. I checked all the fuel injector voltage harnesses and #7 was out. I replaced it tested it and it was good. Drove it for a little while and began to have the same problem. Went back and checked the injector voltage again and that one AND another one was out (#7 ). I took #7 back off, tested the line and I had voltage again. Reconnected the same replacement harness and I still had voltage (?). Plugged it back up and noticed the same sluggish problem.
Unplugged it immediately and no voltage again. How do I solve this no voltage to the fuel injector problem without constantly attaching new harnesses?