Gear problem?

Hello, everybody.

I'm beginning to worry that my "Theo" has a serious problem. The phenomenon has appeared recently, so I thought it was a one-day fly, but today it was back to that, so something doesn't fit. Here's what's going on:
I'm driving with about a quarter of my life. ==References== Drive level. If you give such a slight gas shock now, the motor turns up for a short time (open converter coupling) and then goes gently and slowly again on approx. 140-1500 1/min. With me, it has happened several times now that you notice with this light gas shock that the engine is much more strenuous when turning up. If you then walk off the gas, the speed suddenly down to 1200-1400 rpm again. This also creates striking noises from the direction of transmission/converter?? You also notice that you have to give much more gas to accelerate in order to get ahead. If I switch to Neutral, the phenomenon of the strained engine is immediately gone! That's why I feel like when something in the gearbox suddenly runs pretty hard? The gear can be switched but wonderful, gear changes are gentle and the kickdown also goes without problems. The phenomenon does not always occur but only sporadic. At first I thought it might be related to the switched on climate compressor, but today it was never on and the problem arose. You can also notice this when driving with a closed converter coupling from approx. ==References== There is this jerky speed drop even more clearly to feel, since now the "damping" effect of the converter is no longer there!
Could that possibly. have to do with the oil level in the gearbox that it is too low? Or does anyone know this phenomenon from their own experience? I don't believe in an engine problem, otherwise it wouldn't be gone immediately in N...It has something to do with gear and load.

Thank you for your help!!

Greetings Luke