A few ideas / questions:

1) Are you using a Keystroke actor - or - AppleScript? If you're using a keystroke actor, what are you settings? (preferably post a screenshot showing all of them, f.ex. on droplr.com)

2) How have you mapped that action? To which button? Is it the only action mapped to that button? Or have you, f.ex. mapped another action to its "hold" version?

3) Just as a reminder: the Plex profile is technically broken and shouldn't be used with Remote Buddy. For some buttons, the Harmony will sends the codes of multiple button presses for a single button press.

4) Does quitting EyeTV make the problem disappear?

Best regards, 
Felix Schwarz