My 85 380SE did the same thing when I purchased it last year. I ran Techron through twice and now the gauge works perfectly. My car was not driven very much by the previous owners and I think deposit buildup on the sending unit causes it to work erractically. My gauge would jump around after it reached about 3/4 level.

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95 E420 ( 76K) SOLD
85 380SE(125k)
95 SL500(72k)