When you replaced your wp did you replace or disconnect your temp sensor wire (brass plug next to thermostat). Maybe it was loose before? My gauge swings between the bottom 1/3rd and 1/2 way point as I drive when the thermo opens/closes cooling the engine.
This is the test from one of my earlier posts to check the gauge: per this thread
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As far as the gauge, it's more likely the sensor under the hood, rather than the attached gauge in the dash. The sensor in mine is threaded into the theromstat housing. Looks like a small brass sparkplug w/ electrical spade lug connection on top of it. Single plug wire attaches to this lug. I recently replaced my thermostat,(95 EB) and was able to feel the underside of the little brass sensor plug in the thermostat housing. Someone here recomended turning the key on, then removing the wire going to this brass sensor, then touching to a good ground. The temp gauge should peg to the max heat position, as the sensor is a variable resistance type, and you are now giving it full 12volts. IF it does Peg, it isn't the in dash gage, rather the sensor. I was going to replace the sensor but after LIGHTLY sanding the underside of it when doing the thermo, it started working. Probably better to replace the sensor, as it's about $15 at dealer. Be sure you are getting the correct sensor though, as there is another one that gives feedback to the computer for enging temp. Be sure they know what one you want.