I've experienced the same thing on my Tyros. But I solved the problem after adjusting the Master EQ in the Mixing Console. I used Leo Dunne's "Medium" settings (with a slight variation on a couple settings) - from the PSR Tutorials site which can be found
Here PS: The settings are mainly used in conjunction with the Tyros' Vocal Harmonizer but it seems to have corrected my volume spike problem that I had in the process.
Also you may want to adjust your Aftertouch settings in the Function/Controller/Keyboard/Panel Menu.
I have the Initial Touch set to "Normal" and the Aftertouch to "Hard" on my Tyros.
Hope this helps.
Best regards,
Mike
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Yamaha Genos, Mackie HR824 MKII Studio Monitors, Mackie 1202 VLZ Pro Mixer (made in USA), Cakewalk Sonar Platinum, Shure SM58 vocal mic.