weird 300e antenna behavior

I had Becker fix the radio in my '89 300e a couple of years ago and replaced the power antenna with a used one about six months ago because it was not
working at all and it has worked fine up until a few weeks ago when it started to go all the way up for about 10 minutes when the radio was first turned on and then it would go all the way back down, completely on its own, with the radio still turned on.
I'd turn the radio off and then occasionally try it again later and sometimes it would go up for a minute or two and then go right back down--usually it would not respond at all. It's almost as if it wouldn't stay up once the radio got warmed up.
So, today I checked the connections on the radio, with the radio on and antenna up and I couldn't get the same bad behavior at all--maybe didn't wait long enough and the car was obviously parked. Pulled the radio out completely to check all of the connections again and this time when I reinstalled it the antenna will only go up about a foot. I also checked the connections at the antenna and they all look fine.
Any ideas what I might have done and/or what I can do to fix it?
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