Sometimes faults are heat related. After the amp has been on for a while components heat up and their tolerances change. For example a 2500 ohm resistor will change in restistance as its temperature gets hotter or colder. This is why you will see on electronics items spec sheets "do not operate under 2 degrees or over 60 degrees for example. The amplifier obviously has a faulty component which is playing up when it's warm. Which is probably the exact same problem we have found in them before. The electrolyte inside the capacitors can dry up causing heat related problems and dry joints can cause the same. Unfortunately any heat related dry joints are usually time consuming to find hence an uneconomical repair.