This is going to be a roundabout answer, because it is practically impossible to know if there is something wrong without going through a conplete tuneup/service. There may be some shortcuts if you already know that valves are adjusted properly, ignition is set and operating properly with new spark plugs, with new fuel and air filter, etc.
What I suggest is that you download and read the factory publication commonly referred to as the 'Blue Book' ... in its entirety before deciding what to do next. It is available here in the Pelican Technical Articles section at the following URL, along with other resources on MFI:
http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm
If you go through a complete, total service, then adjust the MFI, I'm sure you can get your engine to idle normally at 900 rpm. If after that, your idle speed still drops with the lights on, then I suggest using the hand throttle to compensate when you drive with the lights on ... that is what I do when operating my air conditioning, and I really don't think the idle should be messed with if properly running at 900 rpm with no load, fully warmed up!
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Warren Hall
1973 911S Targa