What you are really suffering from is Acid Rain which has etched spots into the clear coat. The only real solution is to get it professionally buffed out. However, you can reduce the problem yourself by hand with a mild cutting compound used a number of times very carefully. Be aware though, this may not entirely remove the problem.

My suggestion is to use Meguiars Clear Coat Preparation, then one of their mild cutting polishes, then a high Gloss Wax. I bet after that you will have to look real hard tofind the problem. Ask me how I know!!!

Hope this helps,

Geoff
'94 525iA Diamond Black
Drop Dead Georgeous (And the car's not bad either... :)