[Blas]

The word "accurate" and Smiths should never be used in the same sentence. But remember, you are driving a "car" not baking a "cake". I would think that a temperature gauge should register within 5 or 7 degrees. If your is acting as you said, it is on the way out. There is a expanding gas inside the capillary tube, if a spark from a spark plug wire should hit this tube, microscopic holes will allow the gas to escape. As it escapes, the readings on the gauge deteriorates. Sounds like what you are experiencing. The only recourse is to replace the gauge. Repair costs about the same as replacement. A great option for vintage gauges. Replacemed is not too hard, and you can opt for better routing of the capillary tube to prevent a repeat replacement.