I have a pretty basic empeg install. Empeg Mark 2a connected to a 4 channel amp in the trunk. In turn, the amp drives 4 speakers directly, and no sub. The two front speakers were bought at the time of the empeg install, while the 2 rear remain the stock ones that came with the car. Everything worked fine for about 5 months, but now I notice my left front speaker goes out from time to time. I haven't found any common cause, but when it happens, turning the empeg volume up about 10db will bring the speaker back to life, even if I then drop the volume to nothing right after.

Any suggestions on what to check first? Being that raising the volume brings it back, I don't believe it is a connection issue, but I have checked all the connections that are easy enough to get to.