You need to top up your charge. The 300E system with R12 is one of the coldest A/C systems I have ever experienced. My '89 will generate frost on the center vents on a humid day. Check the sight glass (right front engine compartment) on the receiver/dryer. I'll bet you see those "tiny bubbles"..

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325