This may be stupid to take a look at... but just read.
I had a 2000 Daewoo Lanos, and part of routine service you have to have the valve cover gaskets replaced becuase they dry out and wear out. Now, on one of these services these suckers are easily messed up upon installation. The main thing that I noticed was not oil leaking or something very drastic.
Instead I noticed that the engine temperature gauge would go up and down. I suppose it was from oil or something leaking and cooling the block on the outside. I really never investigated it or anything too deeply since the shop that did the service had to redo this service free of charge... Plus the car was too small to work on.
I would suggest for looking for any kind of fluid leaks of any kind, just becuase there is a temperature problem does not just mean that it would be in the cooling system itsself. check around all gaskets where oil could leach, and check oil level. Also note if it happens ALL the time, or only in wet climates If it is just during wet times, maybe it is the water from the road just hitting the engine right or something.
In addition to this, check your fan clutch if you have a mechanical fan. It could be possible this part is not functioning properly, there for your fan would be changing speeds, changing cooling speeds of the engine eventually.
It is a more abstract way of looking into the problem, and is really just a guess to me. Best of luck to you, these problems can be very fusterating so just stick with it!
2005 Ford Ranger SuperCab XLT 4x4
4.0L V6. 4:10 Gears. BFGoodrich Mud Terrain T/A KM's, Torsion Bar Crank, Bilstein HD Shocks, Autolite Platnium 5143 Plugs, Autolite Professional Wires, Tonneau Cover, Bed Liner, Chrome painted rear differental cover... Thats it.