420 SE ('89): low idle + 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 800 in N and 600 in gear (without having done
anything)...!! Now the car is stumbling at this idling speed of 600rpm 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?? :confused: NB Transmission has been revised completely in june 2004.
Thanks very much in advance!
Rmy K. 420 SE (1989) 313000 kms
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Could well be due to a vacuum leak. The shifts are modulated by a vacuum powered modulator on the transmission and vacuum only comes from one place - the intake manifold. The MORE vacuum applied to the transmission the softer the shifts will be, conversely NO vacuum makes for very hard shifts. So a vacuum leak somewhere between the intake manifold and the transmission shift modulator will cause the shifts that you describe.
Back to the engine. A vacuum leak will dilute the air / fuel ratio to cause a poor idle (which someone may have tried to overcome by raising the idle or adjusting the mixture. When the vacuum is not leaking the engine runs too fast but the transmission shifts well but when the vacuum leak occurs the reverse is true. Find the vacuum leak, fix it and then have a shop set the idle to its correct level and it will be fine.
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