Assuming an ignition fault, check for broken wiring to and from the ignition coil and distributor. if all that checks out, test ignition coil if coil tests ok, (coil may be breaking down under load so a good test can be misleading). Test ignition module and pick up coil.
Doosan's have many wiring issues especially the -3 units. So check the wiring harness crossing over the transmission , wires often break under the clamps.