I had a look through some MB bullitens and found something that may be relivent.

It's refering to the clutch pedal "sticking" occasionally, but It could mean "sinking" if it has been lost in the translation.

It's refering to "thermal overload on slave cylinder (formation of vapor bubbles)" which sounds alot like your problem.

Remedy:
1) remove clutch slave cylinder
2) remove clutch line
3) remove shield if present (I think it means heat shield)
4) install insulated slave cylinder
5) install insulated clutch line

Part numbers: Clutch line A210 290 28 13
Slave cylinder A210 290 00 11

This bulliten (AR29.10-P-6130A) applies to 124,129,140,163,170,201,202,208,210.

I hope this is relivant to your problem, check the part numbers with you dealer incase they are different between LHD and RHD (i doubt they are since no referece is made). Get back to us if it solves it.

__________________
Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32