I have for the last three years used Tomtom with similar problems.
Try this one: Zoned speed 60 as indicated on unit, then it reverted to 50 when the signed speed displayed 80 and held this during a period where the designated signing moved to 100, still holding at 50, and did this for a further 800m when it woke up and displayed the correct speed limit 100.
BTW this particular piece of road has not had its speed rating changed for the last four years Its signed 60, 80, 100 and clearly so. For recent changes I'll wear them but this is !@#@$# stupid for a unit that costs what it did.