Thanks Mark and Alan for excellent info.
I can now understand why I have one 'hottie' in my batch of four PA fets. I am thinking the hot one isn't well matched with the others.
One more thing though; A couple of times I have got up in the morning and looked at my WSPR beacon QCX and noticed that power output as judged by forward power as judged on my twin meter swr meter is only about 20% of what it was when I went to bed and all the PA transistors were steaming hot. I replaced them one at a time and suddenly (obviously hitting on a bad one and replacing it) the PA went straight back to normal, with full power.
Is there a failure mode where one bad one would impact on the good PA fets like this?
I'm sort of thinking that when running wspr on it, reducing the PA stress by swapping power supplies from 13v to 7.5v is probably a sensible idea. 2 minutes key down with a PA which has no heat sinking is probably a bit of a harsh regime to out the radio through.