There is a common issue with the fan contorl module - I have a module that has phantom moments of cutting in and out reguardless of tempreature pretty regularly. I replaced it with a second hand one, and that only controlled the A/C fan and actually caused me to over heat. You can buy a replacement from the USA or local dealer, as a second hand may be no better.
The temp gauge should sit no higher than halfway at operating temp, so that is normal.
Use a torch with a strong white light and look around the plastic on the radiator for any white or green-ish dry residue. Same will be present on hoses or hose ends if they are leaking. To test properly you need a $350+ pressure testing kit - look them up if youre curious. If I was in your situation, I would book into a workshop and ask them to do a pressure test. This will tell you 100% what is up.
Out of curiousity, how do you know to top up the coolant? Are you just filling the radiator, or is the dipstick on the overflow bottle reading too low?