That's my opinion too. My pump was working fine, I ordered a new one on accident. I swapped it on anyway, I was amazed at how much the flow increased in the tank. Could be the same issue here, the pump is getting old and tired.
You can buy the GT500 pump, you will just need to modify your bracket to mount it.
You can buy the GT500 pump, you will just need to modify your bracket to mount it.