Sounds like youve got a major air leak, check all the vacuum pipes and inlet
I've had a couple of different cars over the years do exactly the same
thing, and both times it came down to an air leak.
the first time it cost me hundreds of pounds changing coils and ignition
modules etc only to find a split vacuum hose on the bottom of the inlet
Quick check tip, I knowits difficult because of the engine noise, but get a
colleague to rev the engine for you while you listen for a strong sucking
just be carefull of the pulleys / belts etc. Also remember Radiators get
bloody hot! Im probably teaching you to suck eggs here but I wish someone
had told me first time!