I also have a lightspeed and a mag set up, and I have had erratic RPM readings a couple of times over the last 12+ years. It sounds like you have your problem solved, but I am curious.....did you happen to turn your mag off when you had the erratic rpm? I have had the erratic rpm issue before (about 2 months ago and about 12 years ago), and it turned out that the lightspeed dsub connectors can get just enough corrosion that they don't make sufficient contact. My rpm alarm went off (it was reading erratic), but the engine appeared to be running fine. I switched off the lightspeed, and the rpm went to a steady/normal reading. I then switched to lightspeed, and turned off the mag and the engine would not hardly run and the rpm was erratic. To fix my problem, I just plugged/unplugged the dsub connectors a few times, and then used dielectric grease on the pins.
Just fyi.....I have a Dynon EMS-D10, so I have rpm indication inputs for both the lightspeed and the mag. The D-10 uses the rpm input of only one source at a time, and it is the one that was selected last. So, when I do a mag check, I always do the mag first, so that the lightspeed will be the rpm source. This way, I know when it may be having this issue.
just another possible solution for others.....
RV7, Aerosport 0-360, Sensenich
Hilltop Lakes, Tx