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I see the following issues or possible problems;
1) That antenna is good for picking up real channels 14 and above or UHF band. Real channels 13 (VHF) and below don't work well with that type of antenna. I count 4 or 5 VHF stations... Are most of your problem channels on 13 or below (not the virtual channels that show on the TV but the actual broadcast channel).
2) The aim. Your broadcast towers bunch at 68degT and 140degT that's a spread of 72deg and usually too wide for a single antenna. If your installer split the difference then you'd get crappy reception from both antenna farms. See if you can tell which direction the antenna is aimed. You probably need a two antenna system but its much more difficult to design properly. As an interim solution and test you could pick one direction or the other then rezero your tuner and rescan your channels.
3) The trees. I can guess that leafless but ice or frost covered branches (this is Canada after all :-) can really mess with radio waves but I don't know. A 30 ft mast to clear the trees seems a bit extreme, might be easier to move it to a different location on the roof. The installer might have just randomly picked a bad spot but the signal is fine 30ft to the left or wherever.
4) The lake. Again, this is outside of my experience or education but if you are very close to a lake I suppose its possible that you are seeing some weird interference patterns due to reflection off the surface and it could be random due to season, weather, diurnal, etc. I'd think it would have to be a fairly large lake though. It is safe to rule out the moon.
5) I'd check the installer's work or have someone with experience with coax check his work. A sloppy or loose crimp-on connector (or god forbid he used screw-on connectors), a stray braid wire touching the core wire, bad balun, etc could cause random problems you described.
I'd do #5 and get the current aim first.
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