I have an old hova bator that is at least 20 years old and all of a sudden i am having a hard time keeping the temp at a constant 99.5 i will check it and it will be 99 .7 then i can come back and check it a couple of hours latter and it will be 98.3 so i will make a slight adjustment. and check back latter it will read 100.7.what could be causing this?