Does the 'spike' effect anything attached via MIDI?
If it does it sounds as if you've a dirty contact in the keyboard assembly.
Note velocity / volume is detrimed by the time taken for two contacts to make. Sounds as if only the last one is being made, and as a result you get max note velocity (127).
Thats my best guess...
Might need a trip to the keyboard doctors.
I've repaired many a D-50 with this problem.
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