Hi Bass,

Is it possible that it's the speaker itself? Have you run that center from the left or right channel of the amp? (presumably those other channels aren't having any problems).

How do you mean by "by swapping interconnects, I determined that it wasn't the amp"? Does that mean that you sent the center output of the 990 to a different channel on your amp, when then fed the center channel speaker? Essentially trying a different amplifier channel?

You swapped interconnects, and it didn't solve the problem.

How about checking for stray or loose speaker cables? From the amp to the center channel itself, are all connections tight? Is it possible that there's a stray strand of wire that's intermittently shorting between the + and - (most likely red and black) connections, either at the amplifier end or the speaker end?

Anyway, just some ideas for troubleshooting.