2000 Subaru Legacy Car Stalls Once or twice on warm up then

2.5gx auto legacy

I start the car, its sounds fine, I let it warm up for a couple of minutes, no problem. I drive down the road 1km and at the first t junction I get complete engine failure, drops like a stone from 2,000 rpm to dead.

I start it again, sometimes takes 2 goes but not usually. I drive off and its like a new car all day. 1 time out of ten it does it again at the next t junction.

I live in Norway and its the same in all weather conditions -20 to +20

I also have LPG conversion on the car and its the same whether I turn that off or on.

Its been doing it every day for over a year. Problem doesn't get worse or better.

Any Ideas