Water Temp Sending Switch

I am having issues with my water temp. gauge.
I have replaced the sending switch with a new one but after she starts it slowly climbs to about 240 degrees and stays there. I never had that problem before.
The reason why I replaced the switch is because I had been driving her and the temp. showed 190 degrees. A little while later I noticed the gauge had dropped to 100 degrees. I took the lead off and grounded it to the block and the gauge shot over to maximum. As I understood the video I watched this was an indicator the gauge is fine but the sending switch is bad.
I am thinking the new switch is bad. I ordered another one but don't know for sure if the switch is bad or some other problem popped up.
Yes the fluid level is fine and no she did not indicate an overheat problem.

their is issues with replace temp senders check this out

Thank you very much for the info.
I find it interesting that the repro. switches have readings all over the board! NOt very comforting and from a quality stand point I am amazed they aren't factory tested for accuracy.
If I am reading it right, "Wells" makes one that is very close in reading as original. I will give that a shot as this "Echlin" I purchased from Napa runs around 220 degrees!