just use qmetro instead of metro and it should work

Thank you Terry.

Yes, qmetro works just fine.

Can you tell me why it's happening in the first place?

I had another look at your patch and got rid of the bang messages coming from the message boxes, and now it works without the need to use qmetro (you also needed to have a master render bang sent to the matrix, to 'push' the matrix out to the pwindow after your setcell messages):

but to answer your question, I think qmetro puts the bangs into a low-priority queue so that renders to the pwindow are guaranteed to be completed, even if it means making the next bang message wait. In your patch the problem was evidently too many redundant bang messages clogging up the render queue. -- but it's generally good policy to use qmetro when making jitter patches anyway

I see....that is the low-priority queue thing... I always had problem to understand that.... I see now. Thank you.