I have an ESP with EMG 81/60, the old soldered ones, and sometimes the 81 make some strange noises and sound very weak. If I flip the pickup switch back and forth, turn the knobs, unplug/plug the cable, it works again. This just happened a few times so I can't tell exactly what I did to make it work again.

The battery is new and the 60 works ok. I checked the connections, cables, solder, etc and everything seems ok.

Maybe a internal problem of the pickup?
Sounds like the jack. You'll need to clean it. If that fails, replace it. They use a stereo jack.
Gilchrist custom
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My band
should not be the pickup...really need to check on the jack...selector switch... maybe...but not that likely as the jack