I purchased a central heat and air unit about 4 years ago and it came with a Honeywell VisionPRO TH8000 programmable thermostat. For the first few years it worked without a hitch. Then last fall it started randomly changing the temperature. At the time I was working night shift so I had it set to warm up to 72 at 5 pm and 7 am then go to 65 at 10 am and 7 pm. I noticed that it began changing to 72 anywhere between 2 and 4 pm. I tried changing the time, the temperature, replaced the batteries, everything I could think of and it didn't work. I finally changed it to only have 2 programmable time periods and that worked but was very inconvenient. So I went back to the 4 time periods and again it worked for a while and then began randomly changing the temperature again. I have since went back to working day shift and have noticed that it does the same thing at night. So apparently it's changing on it's own twice a day. Is this thermostat defective or is there something I'm missing?