I'm not saying this is the answer to your problem, but I had a engine lugging problem when I owned my 680 Case backhoe. I replaced all the hydraulic lines and filters, removed the hydraulic tank and flushed it and the sump screen and reassembled everything.
The hydraulic pressure was down, so I adjusted it with a gauge and the service manual. The hydraulics were like new, responsive and powerful, as they should be. A week later I was clearing brush with the loader and broke for lunch. Came out a half hour later, started the tractor and went to take off and it wouldn't rev above 1200rpm, and it did so slowly. I scratched my head, looked for leaks, checked for blocked air intake and filter and found nothing.
What I found after about an hour of searching is that the hose that feeds the backhoe had partially disconnected, effectively dead heading the hydraulic pump. I cleaned the male and female connectors, installed a new O ring and connected the hose. All was fine. Perhaps the hydraulic system dead heading or the hydrostatic system dead heading is causing your problem.