You replaced the wheel motor - how about the hydraulic pump driving the motor?
Is it the same pump for both left & right wheel motor, or do they each have their own pumps?
On mine each wheel have their own hydraulic oil pump & motor.
Sounds like you may need to take it to someone who is...
It will not solve the problem you are having, but I think it will save lot of wear and tear in future if you just slow down the engine some from "full throttle" before engaging the PTO clutch and rev it up to full speed after that!
I replaced worn out clutch last year, after about 1650 hours...
I have 1700 hours in my 1993 721D (it has a Kubota Diesel engine) which we have owned since 2003 and the engine purrs like new!
It depends on maintenance and whether they are abused or not.
It sounds like the fuel solenoid is not holding the fuel on.
I have had that happen during cold weather startup (we need to mow during winter here!), when the battery voltage is marginal but still enough to start the engine!
Speeding up the engine to fast idle (increase the alternator output)...