I have this HP too. This defect is normal, is due to a mediocre design. The exact place of the component that provokes this drift in the small printed circuit, below, it was a metalic 8 pin circuit integrated. I don't remember which was. I'd don't use these instrument for frecuency precision works, it's clear it is don't made for this. Mine is keeped in a shelf, maybe someday goes to the electronic waste facility dump.