Tony, does it lose left side on all channels?
I guess you did check the pan control....

Could be the connection of the output jack...maybe just a bit of cleaning...
I still have this mixer with the band going on 9-10 years..it redlights on the output meter..but still works(sounds) good..

How about the control room outs...does it work okay there...and what about phone out...and headphone...???