If you need emergency amplification, you can use my Behringer Ultratone K3000. Its a bit bulky and heavier but it sounds good with the PSR3000.

Larry, seeing as half the time, the tech is not finding anything wrong, could it be something else ...
The chord/plugs, does it happen after the speaker's amp has been on for a long period ... Maybe the tech only spends 5 minutes with it and that may not be enough time, as you mentioned, the last time was at towards the end of the performance.

If it's still under warranty, I'd try to get Yamaha directly involved. You've had it into the tech enough times and its not getting better, the company should replace it or at least subsidize.
Maybe try another tech too.

Good Luck and let me know if need the Behringer in the meantime.