Pioneer DEH8600 intermitent no sound

Hi guys, if anyone can suggest a cause and/or solution, I would be VERY greatful! As the subject says, I have a Pioneer DEH-8600 in a Honda Civic Type-R (2004). About a month ago I started to lose sound through the speakers (stock speakers, connected via an amp, through a preout). When this happens, the sub (separate amp and separate pre-out) remains working. I can have several hours of music, then several hours of no music (just the sub): There is no pattern I can see. It makes no difference if I switch the car/Headunit on/off - it works when it wants to! I would say that there is usually quite a large time (of 15mins+) where the unit is either working or not. The unit can start/stop working when I'm sat in traffic, so don't think it's movement causing the issue. Checked the power to the amps (fine) and all the cables and connections seem fine. As the speakers are stock, I don't think they are grounding out. Is this a known fault on this unit? Many many thanks in advance for any help/advice you could give.