had my audio stuff in my car since 2006. friend installed it all. just started acting up recently. the speakers work fine but every once in a while the subs go out. there's a deep pop, not too loud and then they stop working. so i'm thinking either the sub amp isn't working anymore or the wiring is sketchy. also, it's totally random when the subs go out. never go out when i go over bumps or holes. it was below average equipment in 2006 so i'm guessing it's just dying. this is the only stuff i have records for anymore. funny enough the subs i have were recommended by someone on d2jsp. and i believe the company is out of business now as i looked them up about 2 years ago and found out they no longer exist. can't remember the name of the company now though =/. but they're 2 8's cause that's all my truck could handle underneath the seat.

KENWOOD EXCELON KDC-X790 CD/MP3/CD-R KDC-X789
PLANET AUDIO 2 CHANNEL 400 WATT MOSFET AMPLIFIER
PHOENIX GOLD X400.1/LPL44-D2 MONO CAR AMP

ew@your amps.

aside from that its going to be the amplifier if you can't find any problems with the wiring.

the popping is probably voltage spikes and they stop working because it goes into protect mode most likely.

This.
I had a problem with the wiring in a setup I had a few years ago. Just rewired it and it turned out fine.
It was the same problem.. Popping sound and they go into protect. Actually blew a 12" because of it.

sounds like bad boards on your amps. I know it sounds funny but you should be able to smell the fried heat sinks or boards after they go into protect. If your subs had been running wired at a load to low for the amps to handle they may have worn out over time.
how are your subs wired?
What amp is hooked to what?