309 green 1992 engine dies

Hello ! I have a problem, Engine works very well but sometimes the engine dies suddenly. I just turn off the engine and turn it on again and the engine works as nothing happens. It always starts very easy even when its has been parked for a week. But when driving, this thing happens quite often now. I replaced the fuelpump but with same result. I think its a ignition problem somehow. I can try to put a extra fuelhose and a filter (bypass the orginal fuelfilter to see if thats the case. But my experience about fuelfilters which get dirty , dirt dies when you accelerate but works ok otherwise. Anyone here has an clue or experience of this problem. ?
Thanks , Anders