As obvious as it might sound, there are I think 2 aspects in your question:REV2 and Ableton.
The REV2 might indeed slightly fluctuate and I would believe this is absolutely normal. MIDI clock is just a tick and the REV2 has to guess the BPM which is nothing but a measure of time in between two clicks and possibly a low pass on measure to avoid crazy changes on display.
On the Ableton front I faced a
lot of issues when I had the sound card, the REV2 and other devices on separate USB.
After a lot of experiments, the following setup seems the most robust:
- 1 USB connection for audio and midi
- 1 MIDI hub from the sound card MIDI port to all MIDI devices
- I can then keep other USB connections to any device for non musical purposes (patches)
As a matter of fact I do not use VST editors, I use standalone applications beside Ableton.