Hello I have this pedal too and have experienced similar problems..
If you are using battery power only use Duracell Ultra (or equivalent) never re-chargeable batteries. However I am assuming you are using the mains power adapter rather than batteries.
Start by eliminating external issues. Change your guitar lead and output lead - if you don't have spares swap them over or swap the ends round to make sure it is not a contact issue in the jack sockets.
I found this problem occurred most often when I was using headphones plugged directly into the pedal rather than an amp. I was usually able to solve it by wiggling the output jack plug slightly so came to the conclusion that it was just a poor quality plug or socket.
Having said that I think that there might be a bug in the processor unit itself because I have occasionally experienced the problem when playing through an amp. I resolved it by unplugging everything and starting again but this is a bit annoying either during a gig or half way through a recording session.
Does it do it on every patch or only on some? I have one patch that is completely clean (straight through) with no effects, noise reduction, boost, with the tone set at flat. You could try setting a patch like that up as a comparison and see if it still does it.
Lastly you could try recalibrating the expression pedal.

1. Unplug the input jack to turn the unit off.
2. Hold down the pedal assign button whilst plugging the input jack back in.
3. The display should read 'dn'
4. Raise the pedal fully and press the store key
5. The display now reads 'UP'
6. Push the pedal fully down to the stop and then release it (it will return slightly)
7. Press the store key again
8. Job done (unless the display reads 'Er' - in which case repeat from step 4)
9.