tough fix, but...
I just got done fixing my varying (mostly too hot) temps in my car. The procedure?
Flush coolant, replace with 50-50 h2o/coolant mix. Add water wetter.
Check elec. fan operation and see if air dam has fallen off.
Replace rad. cap and tstat - stock temp'ed tstat should work if all coolant components are working properly.
Check with ohm meter and replace if necessary 1. temp sender for the computer (on top of intake) and 2. temp sender for the dash gauge (by tstat).
Replace hoses (most importantly the two connected to the radiator - make sure the bottom one contains a spring inside of it).
Replace water pump - may not be necessary but not a bad idea for the cost.
Replace radiator.
After doing all of this (~$400) my car runs cooler than most new cars I've watched run. I didn't use a stock temp tstat but plan to go back to one as my temperature almost never gets past the n in norm(this car is driven city/hiway in 80-100 deg. weather). You might also find that as you go through replacing all of these that at some point in the middle your problem is fixed, however, I had to replace the radiator at the end to fix the problem completely. Now I have no temp. variation and the const. temp is fairly cool.
Don't know if there are other things that might affect it but this worked out great for me.
Brian