New V8 fan needs help

Hello, guys! I have been visiting your site for a long time and I find it absolutely fantastic ! The V8 was already a dream of me when I had my rags fresh in my pocket at 18, but at that time it was only enough for a 100 5E, with which I had a lot of fun for 2 years. Then there were 8 more Audis, from the 80s B3 Quattro to the 100 S4 with 6-speed and SMS Revo Tuning.
Now finally my 3.6 automatic is at the door, Model 90. Even though almost 300,000 on the clock, just made like from the egg, black, good equipment, 16 inch and large inspection with timing belt.
However, two problems have now emerged:
After a longer motorway ride (300 km at 160-230 km/h) the automatic suddenly refused to set up the fourth gear.
I was able to turn out the third to just under 210, but then gave up and snuck home with 160.
I switched from E to S in between, but no difference, almost as if E and S had identical characteristics at once.
This morning with a cold engine everything was ok again and the fourth was inserted without any problems.
Does any of you have any idea where the mistake might be?
The second problem annoys me a little more:
If I want to start the lukewarm engine, the machine starts immediately, but then the speed drops immediately into the basement and in most cases the engine dies off.
Then have to keep the machine running with gas and then can drive without any problems, as if nothing had happened.
Just don't take your foot off the gas...
Even if I switch to N at the traffic light, the speed is sometimes slowing down, but then usually catches up again before the engine stoppage and levels up again at 750 to 900 revolutions.
I am very grateful for every tip !
With the best of regards,
Michael