CAT M315C transmission pressure low

Hi from Finland, my friend likes to ask help The problem with his CAT is, that very often, randomly, right away or after some work (operation, digging), CAT tells ”transmission pressure low” and stops digging. Today it happened right away, we have checked pilot pressure and it was ok before that. After the situation we changed external gauge (noticed 2bar difference between external gauges) and add some pressure between specs limit and it worked approx 15mins. So pilot relief pressure has adjusted to exact 33.5bar and reduce pressure to 31bar, middle between limits. Also checked with 1st and 2nd gear pressure is exact same in the transmission 31/0bar and 0/31bar depending gear choised. When digging, brake is on and transmission pressure should be and is 0bar. Could this be electric / program / sensor / other sensor failure, cause sounds little weird, when digging and transmission pressure should be and is 0bar, it announces then the failure and digging stops. How reliable the external gauges might be, there was approx 2bar difference between the ext. gauges we used (other told 29bar, other 31bar). How to be sure there is enough pilot pressure at the moment, can CAT tell that some how? Has pilot pressure so exact specification limits 30-32bar, If little less than 30bar, will there be failure announcement right away and digging stops? Oil temperature was colder (approx 40C), pilot pressure should be measured between 50-60C. If oil is little colder, guessing there is more pressure, but the original problem also might occurs right away with cold oils, when ext. gauge showing right pressure. CAT’s serial number is: CATM315CEBDM02138 BR, Sami