There could be a whole lot of other possibilities, including the LPG connector not being on correctly (does it have a black gasket/square o-ring in the end that stays on the forklift?, it should not, that is the outside gasket from some tank that lost theirs). Have you tried a different LPG tank?
Is that the correct model number? Can you give us a serial number, or at least the serial number series?
What sort of ignition system does it have, it could also be a bad or poorly grounded capacitor in the distributor, if you have points. It could also be a burnt up external resistor if the coil's wiring has an external resistor.
When it dies out, is there still power (battery voltage) from the ignition switch to the coil positive side?
what sort of LPG system is on this unit? you could have a bunch of "gummy gook" inside the regulator. Have you tried to drain the regulator?
Have you done a -complete- tuneup?