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?

The second one its dieing out after a few minutes running but there is no freezing of the regulator

but is the coolant flowing through the regulator?
a rusted stuck engine coolant thermostat, or a LPG regulator water line thermostat that fails to open at it's rated temp, will cause freezing up of the regulator.
Or are you saying that it still is dying out after a few minutes running but there is no freezeing of the regulator?

Fresh antifreeze and its new just been put in.

I put a new regulator and it is not freezing up.

does it have plenty of coolant in the radiator? is the coolant able to circulate through the LPG regulator/vaporizor? is the LPG regulator freezing up?