Many possible mechanisms can create temperature fluctuations on the scale of an hour or two. First there is the met site itself. Are there trees or buildings that could provide partial shade, especially when the sun is low in the sky? Are there buildings that could reflect sunlight directly onto the instrumentation?
Then there are local anomalies. Are there gully winds? Do cloud banks form in the mornings, or high cumulus towers build up in the afternoons (quite common in the Western Ghats of India)? Is there a local temperature inversion, and does it disperse daily? What kind of local wind patterns are there? Is there a lake nearby that could create local cooling if the winds are in the right direction? Conversely, is there a source of local heat anomalies - factories, power stations?
On the larger scale, what is the area topography, and what local air circulation does it create? Is there some interaction between monsoonal air flow and continental air masses? What turbulence does this create?
Some bizarre anomalies are also possible, like birds roosting on the instrumentation!
Really, there is no short-cut answer. The only solution is to go there with an unbiased and inquiring mind, and check out what's going on. I'm guessing that for such a consistent fluctuation it will be something obvious.