Had exactly the same symptoms as you but mine would also cut out for no reason,then run fine again,I thought I had been accidentally hitting the kill switch,mine was,as overdale has said,the head gasket blowing into the cooling circuit. I was told at first it couldn’t be that,very rare,but it was.
Great bit of kit,we have had one for a couple of years now.Its the double one that can be configured to a single one if needed,we use it to tow two track bikes that are about 190kg each and it takes it fine,we have another non folding trailer for the trials bike mainly to keep the Motolug clean for the track bikes tyres.
