I have a 1999 carrier 58PAV furnace and it is not holding the temp from the setting on the mechanical thermostat. If I set it at 68 it will usually drop down to 50 and finally kick in or sometimes I have to manually re-flip the switch on the furnace to start up the blower. Sometimes this starts the furnace up and other times like very early in the am it will not come on. Not sure if we I should start with changing the thermostat or something else? I also checked the led light on the circuit board and the LED light is not blinking continually but it pulses. Not sure what that means either. Also seems to run fine throughout the later morning hours, really a problem at night. Wondering what to do before we call for a service guy. Any help asap would be great - cold Jan day in Chicago! Thanks for your help.