So, I've not been having very good luck lately. 1980 GT with B21/K-jet. To explain a little of what has been going on. When the tank is full with fuel it runs good (I still think it's low on power). Once I get to about the half way mark on the gauge I start having issues. It's as if I'm out of gas. I know this isn't the case as I'll go to fill it up and it'll only takes 5 or so gallons. So, I've pulled the intake pump. I checked the rubber hose and that seems to be in good order. Also verified that the pump is working. It seems to be working just fine. This is a replacement pump from the junk yard as the pump that came out was toast. Just last week I did replace the filter on the pump as it was pretty dirty. Once the car is down to about a half tank of gas the inline pump will start making noise as though it is struggling for fuel.
Now, for the strange part. When down to about half a tank and the car runs as though it's not getting enough fuel, I can park it for a while and than start it up. It'll run fine for a while before starting to act up again. I don't want to just throw parts at this as gas alone is breaking the bank. Could the in tank pump just be weak and not pumping enough fuel once it's a little low on fuel and possible heating up? Could the inline pump be not working correctly? It all seems odd to me since when I let it sit for a while it will than run okay.
Also, the car seems underpowered (used to my 1986 B230 so may not actually be) and getting horrible fuel economy (15mpg). I thought I would add this in as I'm not sure if it will be helpful. Oh, one more thing. I don't have one of them fancy fuel pressure gauges but, I did check pressure at the filter under the hood and I'm sitting right around 120PSI.

Sorry this is so long, I'm just really getting frutrated and need to get this car fixed!