We had a similar problem on a race car one time. Had el-cheapo type $10 gauge in there. Car owner got a "deal" on some Stewart Warner gauges from a sponsor, good deal rite? WE installed gauge pack and ran the car a couple weeks. Started one week noticing the temp was really getting high. We thought had a cooling problem, changed radiators (twice) Finally one day while sorting the worlds problems also with some cold beverages I said well heck lets check out the "old" gauge. Everything worked A OK. Turned out the capillary had gotten pinched when installing as someone ( I wont say who but they dont live with me) thought they had a better route to run the capillary. Just took awhile for it to get shorted or whatever happens to them. Unfortunately there is no way to fix them I know if.