First post here: what's wrong with a crazy DD-5

[DS-1]

Hi all guys! This is my first post here and I'm very proud to be in this amazing forum! I have a problem with my "brand old" Boss delay. When I plug DC or battery on it, it starts normally, but after 20-30 seconds comes the funny thing: the "MODE" pots goes crazy on 5 to 11 position. In few words, all the modes don't match with the numbers. Example: HOLD is not anymore on position 5, but is on 6, REVERSE is on 9, ecc.. The positions are randomized at every start and I really dunno what's wrong with it... can anyone help me to recognize the problem? Thanks in advance!!!