I've got the same problem - because I forget to keep the rollbar clean. (Bad Susie!) That's the culprit.
I'm thinking something very basic, like Simple Green, or maybe something heavy, like 409. Soap and water? It's just dirt and dust from the roll bar, so can't be that complicated. When I get a round toit, I'll report back.
....(looking for a round toit)........

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