1996 Other Land Rover Models missfiring

Electrical problem
1996 Other Land Rover Models V8 Four Wheel Drive Automatic 160.00 miles

I have changed the cat, coil, ht leads, and plugs.
When starting is firing on all after driving for a while will suddenly misfire then after a while will clear it's self then run fine for about 10 miles then the same thing happens.

It will do the same on petrol as it with LPG