88 Camry Temp Gauge Problem?

[mb571]

88 Camry 3S-FE engine:
I have a problem with my temperature gauge intermittently bouncing to full scale pinning the needle. When it went full scale, I could pull the single wire off the sending unit, and the gauge would drop below zero. But once when I pulled it off, it dropped to half scale. I am wondering if I am dealing with two problems. I have replaced the sending unit even though I wasn't convinced it was the cause. It still intermittently pins the neelded at full scale. Is there a likely spot where the single wire is getting grounded? In my crummy Haynes manual it shows three wires going to the gauge. What is the third wire for?

Mark