Could be a big vacuum leak. The car is old enough to be entitled to brittle, broken vacuum hoses. When I pulled the head on my '89, I had to replace most of the neoprene hoses and some of the polyethelyene tubing. It was brittle and falling apart. The shaking engine can't be helping.. Just my $0.02.

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Jeff L
1987 300e
1989 300e
1987 BMW 325