Please help Mercedes problem.??

I have a 93 Mercedes 400 SEL..... It had a messed up injector harness and was running on about 6/1/2 cyls. We got that fixed and it is running on all 8 and is smooth as can be. Now the only running problem is at idle in park or N. It starts and idles fine for about 10 seconds, then the idle speed goes down and up....back and forth about 500 rpms.... It does this until you turn it off or put it into R or D. The idle is perfect in gear. I aslo seems to be much better with the AC on. I don't know where to start at all on this one. I just stand there and scratch my head under the hood of these German cars. Any help is greatly appreciated. Thanks, snipped-for-privacy@aol.com