Yet another fuel guage problem...

[Longhorn Man]

Seems we have been having a few of these lately huh?
Now, I have yet to troubleshoot this, and in all reality, I may not get to it for a while... I have more pressing things on my mind, and more important things to fix on my truck...esp a hand full before the snow flys.
Anyways... on to my goofy POS.
At first, back in the day of the Pontiac 350, and then into the caddy 425 days (or day to be a bit more acurate) the fuel guage worked fine.
The Cad 500 got tossed in there, but nothing was noticed/not noticed right away due to a couple problems.
I did end up removing the tank, thinking it was filled with peeling POR15, which I was wrong, the fuel pump (electric) crapped out on me, however, there was nothing chainged, the same tank and same sender was reinstalled...and the tank IS grounded fine. (I am very anal about my grounds)
Also, at the same time, I removed my inst panel to replace my aftermarket bum mechanical temp guage (ever seen a mechanical one goe bad???) and installed an electric aftermarket one. The printed circuit board was looked at, like I always do when the inst panel is in my lap. I've seen too many with burnt lines on them to just ignore them.
Full is at 1/2 tank, and the lowest I have run it was to 1/8 tank, but I filled up and put in 21 gallons (!!!!!) at that point, so I was 100 feet from running dry most likely. (stock in cab tank)

So, I know there are eleventy billion variables here... however, has anyone seen this before? Just looking for some quick checks before setting aside a bunch of time to look at it.
Oh yeah, and since my speedo cable broke, and I have YET to locate a replacement, I can't just go by the miles driven. I don't know if I should trust this guage to get worse, or if it is going to stay like this. I do know that the top end (1/2 on the guage) has continued to be full.... and I try not to run it below 1/2 tank (1/4 on the guage)... makes it feel cheaper when refueling.