Canon IRA 8295 booklet finisher N1

[sx633]

Hi all, any thoughts on this one ?
I've got an irADV8295 with booklet finisher N1 attached.
Sometimes (not always but often) after a booklet saddle stitch job the machine will continue in a loop of trying to make booklets.
No paper there, and the job is finished but it just carries on even to the point of operating the saddle stitch staplers. The staples just falling into the bottom.
The only way to stop it is switch off and on, then on restart it gives jam code 1387 indicating 4 different sensors, but obviously nothing is there.
Switch off and remove power to finisher for a minute, then power it all up again and all is fine till next booklet job.
I've replaced the edge stopper assy various motors and PIs, and main finisher controller. I am waiting for the saddle controller to be delivered but am not convinced this will work.
Any ideas ?

[sx633]

Turned out to be the photointerrupter sensor which detects the folding roller (The big fat one).
Bent slightly out of position.

[vigour]

Good find pal