Check any and all filter,cross-over,and de-coupling capacitors in every circuit especially on the left channel side,but check all capacitors anyway on both channels,as one channel can affect both left and right channels, also a capacitor can heal itself over time and do the same thing again later,"Cold-Solder" joints can do this also, on a printed-circuit board, HINT:heat and cold can cause problems so,try heating, joints,caps,etc. or buy a can of freeze mist(to cool components) at an electronic parts store and gently heat or cool each suspect component waiting for the outcome,or wait till the condition starts and then try the cooling,because the component has already been thermally warmed up by the circuitry power