What are the visible symptom details when it "stops working and crashes"? Is the display panel blank? If you switch to a programmed memory channel in the 6M band instead of using the UP band button does it still fail? Does it always do this or only after being used for a while (heat related?).
In UHF does it have the same problem as when going to 50 MHz? Your sentence says "same problem" but then also "nothing dead everything works". I assume you meant "everything dead, nothing works" maybe?
It sounds like a Logic board problem or flex cable problem. Has the unit been modified or serviced recently? Have you tried doing a CPU reset (page 25 of English user manual, erases all memory channels)?
Your English is much better than my Italian!