I know this is a old posting but I though I would post to it anyway just in case someone searches to it with the same issue. I also bought a FW120H that had the same symptoms with the effects turning off and on randomly, volume jumping,  and the 3rd channel would stop working as well. I noticed that the leads from all the pots are crimped at the pot its self so that could potentially create an intermittent connection and intermittent issues described in this forum. I took a 27 watt soldering iron and lightly soldered both sides of all 3 crimp locations on each pot itself and not at the board and it solved all my problems but one which appears to be a bad IC. I found the connection issue by powering up the amp with the board out where I could flex it looking for cold solder joints and cracked PC strips. I noticed if I held the board firmly and flexed the pots the intermittent problems would disappear. Now if you are not experienced with working on amps with power applied, do not try this because it is a serious shock hazard. Let me repeat, do not do it if you are not experienced with that form of troubleshooting with power applied. You can do all the soldering with power not applied and just look for positive results after its reassembled. Either way good luck because it is a good sounding amp for the price. Got mine for $120