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It's not odd at all. Jointennas were discontinued long ago, probably 10-15 years past. It just took a long time for the supply to dry up since they were slow movers. Besides, you cannot use one in your situation as your have TWO non-contiguous VHF channels (8 & 10) to deal with.
The 7697 is ill-suited for reception of your San Diego stations (beamwidth too narrow for the spread of your UHF and VHF towers) although it should be a great choice for pointing at LA. Use the RCA preamp on this setup. You would be better served for your locals by having gone with a two antenna setup as suggested several years ago.
The C4 *should* perform well for your locals that transmit from Tijuana and Mt San Miguel but reception of the VHF stations from Mt Soledad (La Jolla) isn't likely to be as predictable due to them being out of design band for the C4. They're probably going to come in by sheer power since those signals are very strong anyway.
Wildly oscillating signal meters are a hallmark of interference on the channel. Could be any of the following: Multipath, over-amplification, FM interference (usually affects high VHF), local electrical or electronic noise source.
Your TVFool plot is lying to you in that it's not accurately accounting for the ridge line to your south. Reception of your UHF signals is going to be substantially more difficult than the plot suggests due to terrain shadowing. Since I don't have your precise coordinates, I can only estimate your location. My estimate is that you're around 610-620' ASL while the ridge line 1/4 mile away is around 705-725' ASL (not including structures). That's a fairly significant amount of depression in that short of distance and it's likely attenuating your UHF signals by as much as 25-35 dB from the forecasts.
Before making any suggestions, please fully describe what is hooked up and how it's all connected and configured.
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