Controlling Zoom RhythmTrak RT-123 ; dropped notes

Hello, this is my first post, hope I'm not out of line.

What I'm trying to do it use MaxMSP to control a Zoom RhythmTrak RT-123 Drum Machine.

My setup is:
winXP->Edirol UM-1S usb/midi interface->Zoom Drum Machine

I'm using makenote with a short duration (5 ms) to send signals to noteout on the correct channel.
I'm able to make the drum machine play this way, but the problem is:
With tempos < metro 800, some of the notes are dropped. Maybe every third, sometimes every other note, or every 4th, it seems arbitrary. At very high speeds, I might get several notes in rapid succession, then a space.

Other things I've tried:
-Controlling the drum machine with a keyboard: When I do that, I can play as fast as I want without problems.
- Controlling other gear: Just by plugging the midi cable into my Novation Synth, I can control it with max (using the same patcher), and get notes out at any tempo, no problem.

So, other controllers and recievers work fine, but something about controlling the RT-123 with MaxMSP is resulting in dropped notes.

Probably no one else is using this exact combination of gear, but maybe someone can advise me on other things I can test to isolate the problem? I'm not very experienced in midi internals.

Thanks for listening, I hope to be contributing to the forum in the future.

don undeen