1998 Volkswagen Sharan engine stalling

Driving down the road with no problem, after about 15 to 20 minutes, the car will falter and revs will go from 30 to 0 a few times, then car will cut out. When this happens it will not start for about half an hour or more no matter what you do, the rac failed as well, it will not fire at all. I have had a new fuel relay replaced 3 times, this will make the car go ok for about six months then it will start again. It only seems to happen when the engine is hot or if the weather is warm. Sometimes the car will drive for miles with no problem then back to square one. Until eventually it will happen more often then go altogether, when this has happened i get a new relay again. But with the cost of the relay why is this such a problem, as it is an intermittant fault the garage cannot find anything wrong with the car when it is going,emissions perfect, running perfect so it is a mystery.otherwise the car is in a good condition for age. It has only done 169,00 miles.Starts first time in all weathers, has gone through an mot with no problem.