Printing stops for no apparent reason  

Printing stops for no apparent reason

5 tool XL running firmware 6.2.0+8816 just randomly stops printing.  I suspect that it is at a colour change but I don't know how to exactly confirm that. The printer seems to be connected to PrusaLink.  That is I can pause and try to resume but the resume never completes.  Tried with a freshly formatted USB and with OctoPrint and it is the same, the printer is running but not doing anything.  The OctoPrint console just repeats the loop:

Recv: T:204.00/205.00 B:60.00/60.00 C:-30.00/0.00 X2:36.00/36.00 A:55.93/0.00 T0:199.00/200.00 T1:25.00/0.00 T2:204.00/205.00 T3:64.00/65.00 T4:200.00/200.00 T5:6.00/0.00 @:64 B@:0 HBR@:81 @0:42 @1:0 @2:64 @3:8 @4:50 @5:0 B_0_0:45.00/45.00 B_1_0:45.00/45.00 B_2_0:45.00/45.00 B_3_0:45.00/45.00 B_0_1:60.00/60.00 B_1_1:60.00/60.00 B_2_1:60.00/60.00 B_3_1:60.00/60.00 B_0_2:45.00/45.00 B_1_2:60.00/60.00 B_2_2:60.00/60.00 B_3_2:45.00/45.00 B_0_3:32.40/30.00 B_1_3:45.00/45.00 B_2_3:45.00/45.00 B_3_3:32.90/30.00
Recv: echo:busy: processing

So it is alive but "processing"

Currently I am running a 4 colour print if that matters.

Any thoughts on how to diagnose this?