Car cut out at low rev's

1.) On turning the ignition on you can hear a wheezing sound for +/- 5 seconds and then goes quiet.(Petrol pump?)
2.) The car starts with out a problem 8/10 times. The other 2 times it sounds as if the enjien struggles slightly before it starts.(As if the battery is slightly run down)
3.) When driving there are no signs of any problems. But on stopping at stop sign, traffic light etc. The rev's drop right down (from 950 to 150 - not constant - up and down - the enjien warning also light lights up) and sometimes even cuts out completely. It starts without a problem straight after that.

Note - on servicing the car +/- a month ago I noticed yellowish powder on the enjien where the spark plug HT cabel seemed to have been damaged. I could not hear or see a "spark" /short between the cabel and tapet cover.

I replaced the oil, olifilter, air filter spark plugs and radiator coolent when I did the service.