Ford: Rev problem 18 zetec Mondeo

I have an 2001 18 zetec Mondeo , the problem is when I put my foot on the
clutch to stop , the battery light comes on as if the car is going to stall,
but doesn't.
The revs seem to go right down then bounce back up to just under 1000 revs.
Any ideas ?

many thanks for any info

could be the idle speed control valve