escort temp gauge responding to acceleration

My dashboard temperature gauge has been acting odd lately. A few weeks ago it started oscillating slowly between operating temperature and a
colder temperature. I didn't think anything of it (perhaps a sticking thermostat?). A week ago I had a new transmission put in, and when I got the car back the temp gauge wasn't working at all. Needle just sat below the 'cold' mark and wouldn't budge. The sending unit was replaced, and now it behaves oddly:
When accelerating, the needle drops to the far left, past the 'cold' mark, and stays there. when you ease up on the accelerator, the needle climbs slowly. coasting in gear or neutral lets the needle come up to the operating temperature. revving the engine in neutral does not affect the gauge (it read the correct operating temperature). Accelerating again drops the needle back to cold.
This means that on highways, my gauge reads below cold all the time, almost. Do these symptoms ring a bell to anyone?
Thanks, Anthony
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