I have a 4 yr old NRF-60 that we use 6 months of the year on a boat. It isn't getting very cold. I dial it down to -11 to hard freeze the items and then back it off to -5. I set it to - 11 and it's just made it to -6 after a day of running. I noticed the machine seems to run warm so I cleaned the intake of any dust which seemed to help. What can I do to get this machine working properly. Thanks