He said he get it serviced every 100 hours so I think the filter is ok. But the pump sounds correct.What is happening is it is getting probably half the fuel it needs under a full load. When you shut it off and then restart, the carburetor fuel bowl fills with gas (this is enough fuel to run for several seconds on full load, more if you're getting about half normal fuel flow). Then when you rev it back to full power, it runs fine until the fuel bowl runs out of gas because the fuel pump is not delivering enough... and you need to find out why. It's either a fuel filter, defective fuel pump, or faulty relay switch.
Pull off the fuel line from the carb and check the flow when you turn the machine to ON and override the low oil circuit (if equipped). If you have good flow, you have an obstruction within the carburetor at the valve going into the fuel bowl. If you have low flow, check flow before the fuel filter and that will isolate if it's the filter or the actual pump.