OK great. You may want to re-reference your EEG data via the process Standardize > Re-Reference EEG > then type in "AVERAGE" in the reference text box as explained in the panel.
I also notice that you have 3 channels than seem off from the rest in the time series plot above. The variance around these time series is very large, which may also be indicative that these channels are noisy and could be declared as "bad". But please try to re-reference first. thanks.