Hi Guys!
I noted too late this thread and some weeks later I went thru the same story and arrived to the same conclusions about heat (I also had strong battles with the 2221 power supply and 2230, that was even incomplete; eventually it seems that I won)
I just wish to add a bit: in my case, even after having added the thermal paste, the problem went on. The cause was that the previous owner didd reverse the fan. In this way the U9208 was heated by all the heat produced inside the oscilloscope, instead of being refreshed by incoming air.
Reversed the fan, the problem disappeared.