Jedi
Make sure you take all components in your system (speakers, cables & all) to the shop. I've seen poor solder joints give weird problems-(resistance of a poor joint can vary with temperature)-Also check your power line voltage. For example-if you have a regulated power supply-with a limited operating range-and your line voltage is close to one end of the power supply range-you could have a problem. It happened to me a while back-I had an 'over voltage' warning light come on a 6 channel PA amp. The power supply 'crowbared' and the amp quit working. I got it to operate by using a very long extension cord and had to plug in about 400 watts of floodlights to get an 8 volt 'drop' over the long extension cord. You could also have an integrated circuit that operates incorrectly when voltages/temperatures get to a certain level.
Consider sending your unit back to a 'factory' repair depot. Quite often, due to design and/or component selection, similar units could develop similar faults-and their Techs may be able to pinpoint the problem quite quickly.