Yep, that Bubba bloke was right, it's always the thermostat.
If the temperature seems to be cycling on and off too often or allowing the
temperatures to vary too dramatically before turning on or off, the
thermostat differential control should probably be adjusted. If this is
beyond your abilities, it's easiest and cheapest to replace the thermostat
(rather than to call a service person). Detailed instructions are usually
included with a thermostat.