I have just bought the Helix LT and have only played with it at home to setup various patches and test a few different ways of using it. I noticed this evening when I was playing that my output level would drop away to half and then would go back to where it was. My unit did this 5 or 6 times in the space of half an hour. Checked leads and guitar etc. and no issue there. Running into some Yamaha HS5 studio monitors direct from the XLR outputs on the Helix LT.
I have just re-updated the firmware to 2.53 (which I was on before) and will try again tomorrow.
I am interested to see if this is an issue or just a little glitch fixed by updating.