The symptoms are just about a perfect fit for an ignition module failure. They don't fit anything else very well. Victa 2 strokes are Bruce's and Joe's territory, and they can give you much better details than I can. Also, before you blame the module, you should try everything easier/cheaper to fix, including the spark plug and the ignition "kill wire" that goes to the carburetor. It is unlikely that the kill wire would ground itself every time the mower gets hot then unground itself when it cools, of course.
To find the ignition module, trace the spark plug lead, or the kill wire from the carburetor backward. Both those wires lead to the module.
If you can lay hands on a replacement module from some abandoned Victa on a nearby nature strip, swapping it for the one on your mower is a good way to test whether that is the source of the problem.
