Based on my experience with that other thread:
- You have to rule the gauge and the t-stat out first. You can buy a little handheld IR gun from Summit Racing, several models under $50. That will tell you a lot about temps if you are doubting your gauge and don't have a backup.
- Remove the t-stat, jack her up and burp her real good. Then start it with the cap off and the tank 1/2 full and see if you have consistent flow. Then put the cap on and see if you have the same temp swings.
At this point you'll have a good handle on your gauge, the water pump flow and whether or not the temp swings are continuing. THEN you can start chasing whether its the engine or the cooling system or both.
And by the way, a buddy showed me his nifty little combustion gas tester AFTER I paid the local radiator shop $50 to test mine. He has been using it for a couple years with good results:
http://www.lislecorp.com/tool_detail.cfm?detail=1332