Astra stalling at de-acceleration and is now virtually undrivable

Advise and suggestions would be greatly appreciated.

I have a 1999 Astra. I bought it new and have kept it serviced. In
February I took it for its 60K kms service (I'm in Australia) and ever
since I've had problems with it stalling. I've seen a number of posts
about the problem and suggestions around cleaning the throttle body,
but I guess I am also looking for a few answers around this.

After the service, it would just stall intermittently. However, almost
always when coming to a stop-sign or roundabout etc, the car's revs
would drop right down and then the car would usually pick up again
before it stalled. I took the car straight back to the dealer who
charged me just to look at the car and couldn't find anything wrong
(and couldn't replicate the problem - surprise, surprise). Following
that inspection, I called them again right away and told them I was
still having the problem and they rescheduled the car for another date
to look at it. They have again charged me even though they couldn't
replicate the problem - again - nor, was anything showing up on the
diagnostic machine. However, this time they suggested cleaning the
throttle body and failing that, replacing it entirely. As I drove off
from the last (expensive) inspection, I noticed the car revving much
higher across the entire gear range (a technician has definitely done
something to it) and now the car stalls at every single
de-acceleration - regardless of slowing down to go around a corner or
even changing back to third gear at 40 km/h. It doesn't matter if its
hot or cold. And if I am sitting at traffic lights, I have to ride the
clutch once I've restarted the engine.

Can anyone please tell me what might be going on with my car? Prior
to the 60,000 kms service I hadn't had a single problem with the car.
The very next day after the service it just all starts going downhill,
and whatever they did the last time they had my car has it totally
undrivable. The dealer tells me it's nothing to do with them and the
initial service, so why did the problem start the very next day? Has
anyone actually had to replace the entire throttle body or did
cleaning it take care of the problem? Could it possibly be anything
else?

regards, Jae.