Transmission downshifting

[Vinalf]

Hi.
My W140 (1992 600SEL) has developed a problem that has me worried.

Over a period of about 3 days, the transmission downshifted, initially into 3rd then 2nd.

What happens is that at a constant speed on a level road with no extra throttle input, the transmission just downshifts for no reason.

Even with the cruise control engaged and sitting on 50 MPH (80KPH) in top gear, the transmission drops into 3rd, then after about 30-40 seconds will occasionly drop into 2nd gear.

It will shift up again after I accelerate and back off on the throttle, but the same thing happens after it is in top gear again at a constant speed.

I took it to the shop and my mech adjusted the modulator, but this has not stopped the problem. He suggested that it might be a small vacum leak???

Any advice on this problem will be appreciated. I have done a search but only found 1 similar problem and no answer was forthcoming then. I have read other posts about a "Bowden Tube" but am still in the dark about this.

Cheers...
Vince