"Audio Gone" question for Audiogon experts

I have an original NAD 3020 integrated amp. I like it very much, but within the past several months sound levels have started to change. When I turn it on, the left and right channel sound volumes are roughly equal. Over the next 15 minutes or so the sound balance starts to change and one channel, the right in my case, starts to get louder, or the left channel starts to get quieter, I'm not sure which. As the 3020 has pre-out and power-in jacks I decided to try the power-in section to see if the problem was in the power amp section. It is not - I have "balanced" sound output when I only use the power amp section, regardless of how long I listen. I swapped out the old power amp sections large capacitors about a year ago, carefully replacing them with the same microfarad values so as not to foul the Q of the amp (still sounds good to me). So, the problem is in the pre-amp section. Could a faulty capacitor in the pre-amp section be the fault? A transistor? What is most likely the problem? Thank you.