A/C Eccentricities- Please Help

[Leeds]

My 91 200q has a new compressor and evaporator, but cool air is 
intermittent.
Regularly occuring symptom: A/C works well for 5-10 minutes, then goes into
defrost mode for a few minutes, then to feet (Bi-Level?) for a few minutes,
then finally back to dash vents.
A/C also stops blowing cold air at random intervals- today we drove 15-20
minutes with the above symptoms (cold air throughout), then the car idled
for perhaps 5 minutes. When we got back in, vents were blowing uncooled air.
Began driving, then turned climate control to "Off" for a 5-10 seconds, then
to "Auto" again, and air began to cool. Switching to "Off" then back to
"Auto" has caused air to be cooled again on several occaisions when system
has ceased to blow cool air.
I've used the Bently quick-check (as far as I'm able without diagonostic
tools) with everything responding normally. I'm guessing the contol unit
anyway.
My questions:
1. Has anyone solved this problem previously?
2. Should my technician be able to track it down via VAG 1551 or other
diagnostics, or is this a diagnosis from hell?

TIA,
Kerry
91 200q with much too warm an interior for my Welsh genes!