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Have only seen this once, Speedo sensor fault, Have a look over the back of the engine and check to see if you have a speedo sensor coming out of the box, seen a few PH2 172's including mine use the same set-up as the PH1 172/ PH2 Cup for speedo readings
Anything listed on the faults?
Could also be wheel spin if you were using it in 1st or 2nd gear
Trust me try it again with the weight entered correctly as the calculations it does will be all over the place with a 79kg weight in there. If it still does it after that THEN start looking for a fault.