Motorrad Blinker blinken nur manchmal?

Thought like the @TheFamousSpy to the relay. By adding “Runker” I came to 3 other approaches.

1. Possibly, the wires leading to the front blinkers are pierced by the many steering movements and could shy at the frame, whereby the plus-guide wire produces a slight short shot with the frame. It can be seen that the flashers then no longer shine so brightly. That’s exactly what I had on one of my Simsons. When the wires hang in the wind, they are pressed by the wind pressure to the frame and thus lay the system lame at higher speeds (0higher speeds).
2. A cable break on one of the wires leading to the blinker switch (here rarely because it is a strand wire. All strands would have to be bent. Could pass maximum through the mechanical clamping/clamping of the cage. Or a wire in one of the blinkers has dissolved and occasionally makes a short circuit due to shaky movements or vibrations that can occur at high speeds.
3. A contact error directly at the flasher switch. A cable shoe or a soldering point has contact only in a specific steering position. The vibrations also match this fault pattern at high speeds. The mimic in the switch itself could also have an error.

Whatever. To fix this, you have to break down the electrical system and measure the wires individually, as well as check the contacts at all components belonging to the flasher circuit (linker, switch, relay, connection points). This is the only way to find a sporadic mistake.

Do you have a schematic? I found one here. Page 47.

https://www.manualslib.de/manual/739184/Fantic-125Cc-2017.html?page=47#manual

It’s badly readable, so it’s best to download and print. If you have little idea about it, find someone who can help you. One of you should also know something about soldering iron. Cable shoes, corresponding crimping pliers and shrink tubing can also not damage.
And in doubt, a workshop has to look for it. That would be the expensive option that could probably cost 200 euros and more.
With the Do-it-yourself method, you will get to know your motorcycle and its vulnerabilities better. And when you buy the next machine, you can use your knowledge.

Good luck.