It could be a huge number of things but you can actually lower down the temp without spending money...
1)Be sure that the readiator is externally clean, road dirt really decreases cooling efficiency and youll be amazed how big is the improvement, get busy with a brush and a low pressure water hose and give it a clean.
2)If th esystem has been neglected (rusty water) YOU SHOULD take the radiator out and hose it for a few minutes and then hose the engine internals too, just remove the thermostat and put to hose on the outlet pipe so the reverse flush clean all the galleries, this helps a lot too, once the system is clean YOU SHOULD ONLY USE A GOOD COOLANT MIXED WITH DISTILLED WATER, do this and youll see how it keeps cleaner longer.
3)Check that the system is properly bled and that the cap is working as intended.
4)If you are going to replace parts dont be a cheapo and replace tstat, fan switch (you can use a Fuego switch that engages at 82 degrees instead of the usual 92 degrees), check coolant pump, its belt and hoses for leaks.
These systems work nicely when clean but if neglected they are a nightmare, it took me the hard way to understand them..
Hope this helped.