Sometimes when I want to delete an unwanted portion (truncate) of the sample [DEL]/[MARK]/[DEL], happens that deletes all the sample, someone knows what this is?

You're saying; randomly, the unit that does what it's supposed to with that button combo on most occasions, will simply completely NOT do what that button sequence is for?

Like if you performed the same thing over and over, now and then, the results change?