I've got a baritone that plays beautifully most of the time but every so often something goes wrong inside and the sound goes muddy and the amp doesn't seem to be getting as much from the guitar. Can you suggest what needs to be checked inside? I did a quick once-over and can't spot any loose connections or bad solders but there might be something I'm missing.

It's basically 2 HBs (one tone, one volume) through a varitone system (bypassed most of the time -- doesn't make much difference, sometimes I feel it's not working.)
Well baritones could sound muddy anyway because the frequency is so low. Check your amps EQ settings.