Fusion reactor do deliver to les power for 1 Fusion heat exchanger
1 Plasma heater do not deliver enough Plasma vor 1 reactor
(also tested without other mods)

I can't make it so that the HX always uses as much as the reactor produces. If you're using less power than the reactor produces, it will back up and start producing power after a while. If you don't want to wait, just let it run for a few seconds before connecting it to the grid.
Heaters to reactors isn't supposed to be a 1:1 ratio. Use more than 1 per reactor if 1 isn't enough.

Sorry but you understood wrong
1 Reactor can not support 1 heat exchanger.
(heat exchanger delivers only 500°C)
can i send you a video (emal adress)?

I think I know what you mean. It only happens without manual recipes and with D-D fusion + T or He3 breeding researched. After some time trying to fix it, I think I'll just make manual recipes default and remove the automatic selection that was before.

I have downloaded the newest Version and the Factorio 1.1.8 beta.
There it works
Thanks a lot.