mk1 golf 1.8 gti running problems

i have a 1989 1.8 8v karmann cabby gti and it runs fine wen i start it up but when it gets warm loses power and cuts out or if i go round a corner fast or break hard it also dies and cuts out as though it has ran out of fuel or drops onto two or sumtimes three cylinders . i was told it was the metering head and injectors but changing these hav made no difference could anyone help me with this problem as nobody seems to know what this can be i have also had head asket replaced but again made no affect

have moved this to a more technical section where hopefully you will get some advice.