Welcome to TBN. You don't list your tractor experience in your profile, so I will start with the basics in operations rather than mechanical problems. Please don't be insulted if you are already aware of these things:
For maximum FEL and 3 point hitch lifting capacity, the tractors engine needs to run at ~2600 RPM to generate the hydraulic flow/pressure to do the job.
If you are not experienced with Hydrostatic Transmissions, I suspect you are are running at too low RPMs and are using the HST pedal like an accelerator. It is really an infinitely adjustable torque adjuster. It you want maximimum torque at the wheels, you must be in the lowest gear range, RPM at ~2600, and the slightest pressure on the pedal - you will move slowly, but you'll be amazed at how much torque you can get.
It is not good for diesel engines to be bogged down so much that they "cut out."
Let us know how it works with these adjustments.