Anyone ever DIY replace one of these? Ours is on its way out, I think the bladder is shot. Pressure is good then steadily decreases, then it comes on and pressure increases again. The well pump itself is providing most of the pressure. This happened once before about 20 years ago so I guess it's had a good life. I'd like to DIY if it's not a difficult job.
Just read your post through more closely so everything I posted earlier is now for information only. You have a well related problem, not a holding tank problem. Think about it. If the pressure is decreasing without a tap opened in the house, where is the water going? With a ruptured bladder, the water would still stay in the tank and the pressure would hold. The pressure is going down because there is a leak somewhere ahead of the pressure tank. Now, I am assuming that you don't have a leak somewhere in the house or in a crawl space. From your description it would be a big one.
You most likely have a corroded drop pipe, or a leaky check valve. If the weather is suited to this there is something you can check out to see if it's something more serious. Between the well and the house where the line comes in from the well, go outside and start stomping around in that area. You are looking for spongy or even springy ground that would indicate a leak between the well and the house. It's possible that you have a leak at the pitless adapter but generally the water is going to go back down the well casing if that was the case.
If you have shallow well with a jet pump, the check valve will be right there with the pump or may be incorporated into the pump itself.