Because of the very tight turn the head cable on these make where it connects to the head the cable is known on these to get broken where is gets folded too sharp to make that sudden turn to the connector. If that is the case you will need a new FFC head cable (which is actually 2 flat cables together). This can also be a problem on the main FFC cable from the mainboard to the slider board. Early on after the JV33 was introduced Mimaki issued a "fix" which was a bit of foam for reduce the strain where the main FFC connects to the slider board and a warning about being very careful when installing the main FFC and head FFC cables to not bend them too sharp because creasing the cables can break the wires.
On ours I had to replace the head FFC cable but I was getting random head error messages but broken wires or improperly seated cables at the head, slider board, or mainboard ends can also cause all sorts of problems as Artbot mentioned. The encoder and encoder strip can also cause problems if the encoder strip gets something on it (quite common) or the encoder itself somehow gets damaged or goes bad (rare). Usually encoder problems results in the printing getting shifted or a full wide band of color, not random scatter like that though. I'm suspecting a cable connection problem myself.