mk3 engine problem

hello people im having some probs with a car im working on for a friend, problem is car starts first time in the morning runs perfectly for 5 mins then revs drop and engine cuts out... then it wont restart unless u leave it so stand for 30 mins, then it runs for a further 5 mins and repeats this process... its a 1.4 8 valve monopoint injection, so far iv replaced spark plugs, plug leads... iv also noticed that when its just about to cut out the fuel injector stops injecting fuel so i stripped and cleaned the monopoint injector but to no avail...

if anyone can give me some advise or pointers as to what to check i would be much appreciative thanks