As per Cliff's guidance...I found that the black wire on the sending unit had worked it's way loose. It's located beneath the thermastat on the intake side of the motor in a pretty direct line to the fan. That explained the strange behavior...reading correctly at idle but going full to the left at revs. At idle the wire still made contact but as soon as the revs went up over 2k the wind from the fan blades pushed it away from the male lead on the sending unit. Once crimped and put back on it's all good again.