engine hunting on tickover

[tom hayes]

I own a 280 E class 1994 old square shape headlamps W124 engine. It has done 81000 miles and just been serviced by local MB dealer.
At around 77000 miles it developed a "hunting" problem with a tickover speed varying from 800 up to 1800rpm when in neutral when put into drive it settles down to a tickover of around 1000 rpm. Under normal driving conditions the car runs very well.
The car has a Signature warranty.
I asked the dealer to check out this problem
Their initial response was to locate an engine management system black box and substitute it...........this did not cure the problem.
They then fitted a substitute throttle Box system........again this failed to cure the problem . The fitter seems completely at a loss
as to the cause.
Any ideas?

A new head gasket was fitted at around 75000 together with water pump

tom hayes