Power indicator lamp problem

[vladogavlak]

I bought a rockerverb 50 combo amp two years ago. It worked fine, but now I have a problem with it. The power indicator lamp doesn't light so much as the amp was new and when I play for an hour the power indicator lamp starts to blink, but blink is not a right word, it starts to change the intensity of light and it seems like the power indicator lamp responses on dynamic of my playing and volume of amp. Sometimes when I play really loud, it lights like it was new. Where is the problem? – tubes, transformer..............? Thanks so much for your time and sorry, my English is horrible.