My guess is that you are using UnO firmware v.1.4. The problem can be solved by ordering an older version of the UnO firmware - v.1.3. The original firmware had a large "threshold" to avoid this kind of problem (possibly caused by excessive noise on the power supply, or by degraded capacitors on the power line which no longer filter out the noise sufficiently). The problem with the original firmware was that the expression pedals only start sending MIDI messages after moving the pedals quite a bit, making the pedal behavior unresponsive. Many complained about that, therefore in UnO v.1.4 that large threshold was removed, but on some units this can cause the issue which you describe.