Hi Peter, all,
What I can think of is to initialize the random seed value of MP (name?) with the same value, every time before you fill a new layer. Another way would be to add the 'random' parameters to your FrameTitle macro. But I don't have much experience with random stuff, so there maybe better ways.
I use now \startsetups[buf1] \startMPinclusions randomseed:=1; \stopMPinclusions \FrameTitle{Overlay1} \StartFrame[width=.75\textwidth] This is the first funny frame ! \StopFrame \stopsetups \startsetups[buf2] \startMPinclusions randomseed:=2; \stopMPinclusions \FrameTitle{Overlay2} \StartFrame[width=.75\textwidth] This is the second funny frame ! \StopFrame \stopsetups \startsetups[buf3] \startMPinclusions randomseed:=3; \stopMPinclusions \FrameTitle{Overlay3} \StartFrame[width=.75\textwidth] This is the third funny frame ! \StopFrame \stopsetups and \setups{...}