how to fix my opel astra?

the van is a opel astra 1.7 litre fuel inj. year 2000 and it has a safety thing on it such that if there is a problem with the van the accelarator will stop working and it will just be at constant revs, the van then won't travel much over 50kph. this is fine but the problem is that this just keeps happening at random times while i'm out driving. if turn the van off and restart it it will run fine again untill the next time it goes.it could be fine for days and then der will be a day that it will just jeep happening. we've had it checked 4 problems and we've changed various parts but nothing has mended the problem. any ideas where my problem may lie?