I had the same issue a few weeks ago, at least the symptoms sound similiar. What happens is I had a very mild leak in one of my heater hoses, very hard to find. Apparently the oil temp gage is more sensitive than the water temp gage as you may have air in the cooling system that will throw the water temp off while the oil temp gage is reading correctly. When my oil temp started to rise I would add water to the overflow tank and that would cause the water temp gage to read correctly (higher) and then both gages would slowly start to cool down. Do a google search with your symptoms and you will find several write ups on it. Hope this helps a bit.