Recently I was given a early 80's Marshall 5210.  Plugged my guitar and started playing.  I really enjoyed the sound of the amp.  After about 10 minutes, the amp started cracking and making popping noises.  Then the amp went quite.  I was not getting any sound what so ever.  I can turn the amp off and let it set about 20 minutes.  Turn it back on and I get about another 10 minutes of playing time before it does it again.  I am not sure if this is a common issue with the model of amp.  I am looking for some suggestions on what it may be.  As well as if there is any thing I can do to test it.

It was a free amp, but if I can get it working, that would be so great!

Thanks in advance for any suggestions you may have.