Heat issue with Lamm M1.2 Reference

I'm having a heat issue with one of my Lamm M1.2 Reference mono blocks or rather a lack of heat.

One of them is suddenly putting out less heat than previously, especially at the rear of the amplifier, where it is almost cold to the touch even after 3 hours of playing.

Neither my wife nor I noticed any difference in the sound tonight, but we were playing soft music all evening so nothing to really challenge the amps.

Anyone experienced anything like this before? Any thoughts on the cause?

Thanks,

Ed