A6 1.9 TDi Automatic

I have a year 2000 Audi A6 1.9 Tdi, automatic. The gearbox have been acting
up for a while and recently i brought it to a garage to get someone to take
a look at it. The cluthes for 3. and 4.th gear didnt lock as they should and
specially this happened when they gearoil became heated. Also lately i
stared to hear sound from the gearbox when cluthes slipped. With this in
mind they replaced the clutch for 3.and 4 gear. They changed a sensor that
was burnt and controlled rest of the parts in the gearbox. I have now been
driving the car for around a week but not been driving longer distances yet
for the oil to get fully heated. That was untill today when I used the car a
little more and on my way home I again noticed that the clutched for 3. and
4. gear started to slip.
Right now i'm a little out of ideas of what to do. I have heard that it's
not very uncommon to have problems with the automatic boxes for these cars.
I've been thinking of replacing the converter but have heard there is almost
never any wrong with them. Another tip I got was to take a look at the
radiator and cooling system for the automatic oil as they say there has been
some problems with them. Maybe try to clean or rinse it. Could it be that
the oil get overheated and cause the cluthes in the gearbox to slip?
There is one more thing that seem a little strange, when the car has not
been used for a couple of days it takes forever for it to get oil pressure
up to the cluthes in the gearbox. Could this be a valve or something that is
leaking???
OiS