An aim point of 128° or 135° is a starting point. Your existing antenna is an RCA ANT-751, it's not very directional so you may not notice much difference in 10 degrees. Try 5 degree adjustments in both directions, reevaluate the quality of reception each time, use the aim point that produces the best results.
You mentioned 80' trees. Trees can produce the symptoms you've described. Just like a roof deck, the foliage absorbs and reflects radio waves. If there is any way to get over or around the trees, the signal stability will be improved. Here in Seattle, strong local signals (with NM values in the +60 to +70 dB range) can be unreliable if the antenna is aiming through a Douglas Fir tree. Wind moving the branches will produce a multipath effect.
To use your antenna gain of 10.4 dBi, I would convert into dBd 10.4 - 2.2 = 8.2 dBd (yes, I rounded the conversion factor from 2.15 to 2.2... hundredths of a dB are insignificant when much of the values used are accurate to half a dB at best.) Next consider NM (noise margin) value of a station you want to receive, I'll use WDTA-LD as an example. Your TV Fool report shows an estimated NM of -0.8 dB. When I add the gain of an antenna with 8.2 dBd forward gain I get, -0.8 + 8.2 = +7.4 dB NM at the antenna output terminals. This is theoretically above the needed level for reception. But the tuner is not at the antenna terminals so we need to consider the effect of the cable and splitter losses, then the imperfections of the tuner. Lets say you have about 50' of RG-6 coax when you total the sections from the antenna to a splitter and then to one of the TVs. That much RG-6 will have about 3 dB loss. Let's also assume you have a 2-way splitter in the system as well, which has 4 dB loss. From the antenna terminal where we start with a NM of +7.4, we must subtract the cable and splitter losses of 7 dB so we get +7.4 - 7.0 = +0.4 dB NM. It looks as if we are still at the edge of reception, just 4 tenths of a dB above the theoretical limit of a perfect receiver. No tuner is perfect so we need to apply a correction factor called tuner noise figure (Tuner-NF). This is a difficult specification to find for consumer grade TVs but generally runs between 6 and 9 dB, the lower the number the better. To use the Tuner-NF, take the intermediate NM at the end of the cable connecting to the TV (+0.4 in this case) and subtract the Tuner-NF. Using a good tuner (low NF) we get, +0.4 - 6.0 = -5.6 dB Net-NM.
A properly specified preamp can overcome the effects of the cable and splitter losses, and possibly some of the Tuner-NF. My experience has been that if the Net-NM is below +10 dB, you won't have enough "fade margin" to deal with intermittent noise and interference.
The CPA-19 preamplifier is specified to have 17 dB Gain and a noise figure of 2.1 dB. In the above example, by applying the amplifier at the antenna terminals, we can discount the cable and splitter losses of 7 dB and substitute the Preamp-NF in place of the Tuner-NF. If you plug all those numbers into
the spreadsheet I've posted, you get a Net-NM of +5.3 dB. That certainly doesn't leave much room for multipath, trees or other sources of interference. You would want to look for an antenna with more gain or look at the signal level at higher mounting levels.