It is a windshield defect.

I'd replace the glass, and go with the 91-93 windshield. It is a direct replacement; no modification necessary. Any glass shop should be able to do it for less than $200. You will love the clarity factor.

Some people say to avoid aftermarket glass. That choice is yours to make. I will say, there are plenty of Volvo windshields that have distortions as well; no glass is immune. Just make sure that the one they put in is satisfactory, whatever the manufacture.

I have had good luck with Viracon and PPG glass, for a couple of aftermarket names.
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chris herbst, near chicago