[Peter Serio]

According to your post it sounds like a bad sending unit to me. The sender I see is very old, and most likely it is worn out. Be cautioned, the market is flooded with cheap smaller sized imported electrical parts, especially service parts for a 50 year old GM car. The new parts are not made to the same quality standards as the original GM parts nor are they tested to perform for more than 3 to 6 months. And those places do not have access to a shop manual (one that is translated into their language) for your car. Most of what is sold now is made 1/2 around the world by people who don't have the car (like yours) & probably never even seen one.
Unfortunately the quality of new parts is not good. From what I have seen in the past 5 to 10 years they no longer make parts that will last; but they will make as many new ones as people will buy. I would recommend seeking out an older NOS, sender. One that was made in the USA.