420 SE (89):low idle and hard gear shifting: linked problems??

Hi guys,

sorry to bother you again:

my idle used to be too high (1000 in N, 800 in gear), but from one day to another it dropped to 750 in N and 550 in gear (without having done anything)...!!
Now the car is stumbling at this idling speed of 550 rpm and from the same moment on the gear shifts very hard up and down: with a shock each time.

I do hope these symptoms are linked to one another: could it be a vacuum leak?? If yes, where should I look then??
NB Transmission has been revised completely in june 2004.
Idle control valve has been replaced in 2003.

Thanks very much in advance!

Rémy K.
420 SE (1989)
313000 kms