Odd Backfire and loss of power

[xredge]

OK here is an oldd one, first off since I haven't create my sig yet here are the details, 94 K2500HD, 350 / 4LE80. I've been fighting with this for a while now but I just don't drive it that much anymore besides towing and the semi short trips into town for stuff, and am gradually working on it, when the pain in my back is tolerable amd have to watch it now that I found out what is actually wrong. Anyways my DD had a mechanical failure that I am fixing so have had to drive this to work which isn't fun on the gas end with a 120 mile round trip. Truck ran great yesterday except the check engine light which when I had it checked was only the egr and that is blocked off. Today however it was raining and pretty dark so after going a little ways decided to turn on the headlights it does OK until about 40 to maybe about 50/55 then starts backfiring sounds like through the exhaust and anything above this it just fights and getting into overdrive is harder finally got it there fo a bit. So decided to just kill the lights and ran great after that, so I'm puzzled at what it can be, don't t hink it was the rain as after I turned them off when through more rain showers without any issues. All of the wires, plugs, dizzy and coil were replaced this year, fuel filter . Alt was checked out last year and fine, but really wondering if I need something with more power even though I'm not runing anything extra right now. I also have a slight power issue and maybe a backfire when taking off with the 30ft travel trailer but once up to speed it is fine. It used to have more issues but replacing dizzy and coil had it almost stopped and just doing it occasionally and was thinking that was just gas related at first which I have plans on replacing late fall or early spring, basically everything but the TBI unit itself. Also the body to ground that is by the heater core rotted away, I already bought one for it but just haven't put it in, but doesn't seem like that would really be it.

Any ideas why turning the lights one would cause the backfiring and power issue, otherwise runs great? No codes except egr

 

[shelbyt.67]

The grounds on these trucks are very important. I would check/replace all yer main grounds ( body2frame, motor2frame, batt2motor). Do that before u start replacing parts. The Big 3 is a good idea too.(larger main ground/power wire)