The heat sink as originally set up was not good enough for full duty cycle modes like RTTY and PSK31 and AM.  It was gel coated and no heat compound was uses.  The key down performance was very poor at less than 2 minutes, and it recovered slowly (the other duty cycle).  With the addition of a 1/8th in. transfer bar, heat compound, and removal of the gel coat, the performance was improved to about 5 min.  When the full size 3/32 heat sink was added, it would "steady state" at 10 watts.  The shape of the transfer bar made the cooling uniform on both transistors, and allowed air to circulate on both sides of the sink.  Don't worry about voiding your warranty making this mod.  See Wayne's response to the problem below: