The problem is mostly likely that the TPU-66A processor needs its software updated to level 01/14.  The problem is caused by an internal buffer overload when operating in areas of high Mode S activity.  Honeywell Software Bulletin SWB TPU66A-SW6 reprograms the TPU with new software to increase the size of the buffer to prevent F8601 faults when the TPU is operating in dense Mode S airspace.