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Stick to your gut instincts with this. If you aint pullin a 2500 pound bass boat and trailer up a 20% grade, then any temperature reading above the M is to high.
If the guage is OK, then you have a coolant flow problem, or a heat transfer problem. Coolant flow can be restricted by a bad thermostat, a radiator blocked by mineral deposits, or a blockage somewhere else in the system. Is your bottom radiator hose collapsing.
This would be unusual, but it could happen. Did you, or any one else, ever add a "stop-leak" product to your cooling system? If yes, has the product gathered somewhere in the system and caused a blockage?
A reverse flush, that is flushing the system out in the reverse of the normal flow, should help. This is also referred to as a back flush. You can do it with a garden hose, but be careful not to excede 20 psi of pressure. You do not want to get caught by the local authorities doing this. The FINE could be very nasty, especially in CA.
If the system is getting air in it, this would cause a heat transfer issue. Is the hose that connects to the overflo bottle on tight, and in good shape? Is your radiator cap in good shape? The system can suck in air at a connection, or get air in the system if there is a head gasket problem.
Try letting the engine run with the radiator cap off. Watch the fluid cycle when the thermostat opens. Is there a lot of air in the fluid, and/or is it foamy? A minor amount of bubbles is normal.
If you suspect a head gasket problem you can confirm it with a compression test and/or a chemical test kit that you add to the coolant. The chemical will tell you if there is exaust gas in the coolant by its color.
Another thing that will cause an engine to run hot is dragging brakes, such as emergency brake cables that will not completely release. Just a thought.
The NORMAL range on the guage is dependent upon the driving circumstances and the load put on the engine. In normal driving, not being agressive or towing anything heavy, the guage should be no higher than the M. (closer to the A) It will be lower when you are rolling, and will rise a LITTLE when in stop and go traffic.
In spirited driving, and/or towing something, the temperature will rise. If you are driving normally, and your temperature is up at the N, then you are at the edge of disaster.
I do not have an easy answer for you, but I can tell you from more than 40 years of experience that you are right to continue to try and resolve this problem. Stick with it! rangerj