Lajos,
One thing came to mind: you're on a laptop, correct? Photo Mechanic pushes the system resources of all systems that it runs on. That's one of the ways it gets its speed. If your MacBook has a modern video adapter in it, it is capable of running in two modes. One is higher performance, but uses more energy, and the other is lower performance and uses less energy. By default, "Automatic graphics switching" is enabled and allows the computer to choose when to switch between the two adapters. Normally this is a completely seamless operation and you won't visually notice the difference. But I can imagine that on some systems, switching between the two adapters could cause the screen to blank momentarily. So here is a possible solution:
Open up System Preferences, then the Energy Saver panel, then turn off "Automatic graphics switching" and then close System Preferences. Then try PM again. Did that solve the problem?
HTH,
-Kirk