This section was the closest I could find to matching my 1996 chevy cheyenne with the 4.3L vortec. Similar engines used if I'm not mistaken?

You can move the thread if there is a more appropriate section.

Anyway, getting some weird behavior from the truck lately. At 171,XXX miles right now. Recently did a tranny fluid change myself (pan contents only) and went to type VI. Worked fine for about 3 months up to this point. Also changed EGR valve about 5 months ago and plugs, wires, cap, rotor about 8 months ago. Also, was having a brake air in line issue so fixed that as well about two weeks ago (symptoms started about 1 week before I did this well pedal feels firm now)

Symptom: I noticed the idle would dip below 500 and act like it would stall when coming to a stop and slightly hesitate when taking off. This morning, I let my truck warm up for a while (bc its cold!) and when shifting into reverse it dipped the worst it had (down to maybe 300rpm) and headlights dimmed then corrected and idled at 1000 for a few seconds.

main seal is leaking (perhaps no relation but mentioning it anyway) and will wait until it's warmer to deal with it.

Excessive tranny fluid wouldn't cause something like this would it? I say this because sometimes I don't get consistent readings from the dipstick (always with engine on and in park on level surface)

So what do you think it could be? No codes thrown at all.