Vauxhall Corsa loss of power, takes ages to 'warm up' before able to drive, revs falling spontaneously and occassionally self revving.

Hi I'm hoping someone may be able to assist. I have owned a Vauxhall Corsa 1.0, yr 2000 for the last 5 months. It has always taken 5-10 minutes for the revs to reach a satisfactory level for me to be able to drive without snatching. In the last week the revs have remained low for much longer and refused to stay high enough for very long, even if the car is 'warm'. I lose power when driving normally and when slowing to a stop the revs will drop and the car will shudder. I have a friend who is a retired AA man who had tried a number of things to resolve this issue however nothing has yet to be successful. An RAC engineer who towed me home yesterday suggested that I needed a new coil pack, my friend is sble to test the input of the coil pack but unable to test the output. My friend is also concerned it could be a problem to do with the ECU as many lights on the dash do not appear at any time! Any ideas??? What do others think???