after month of testing the oddest routines, changing gfx and lots of code snippets i found the soulution.
my run method calls the two important methods, named cycle() and paint() for game logic and rendering. if i put after each call a simple Thread.yield() it runs perfectly.
sure you can assume that this is so easy that i have come to this solution much earlier, but!
i tried to replace sleep(..) with a time controled loop of only yields. i put a yield before and right after the sleep call. i raised the ticktime up to 300ms. all that has not the effect like this two stupid yields !
how can it be that a very long sleep can not replace a simple and much shorter yield !?!?!