Agreed that you may have a problem with TOO MUCH signal strength on the two stations that are breaking-up for you, tho' you didn't mention how close those stations are to you. This is usually a result of the strong incident signal overloading the input of the preamp, or the net signal to the TV's tuner being too strong. It can also be a mix of strong adjacent channels overloading the weaker channels you're trying to receive. You might also have a multipath problem that can result in intermittent complete loss of the digital image. Even if the signal is still strong, the time delay between multiple paths of the identical signal can't be adequately resolved in the tuner. Sometimes rotating the antenna slightly can help with multipath problems.
If you have any analog signals in your mix, you can use them to advantage to help troubleshoot the system as the displayed PQ will vary continuously with the measures you take. But it's tough with the "all of nothing" digital signals.
I agree that you really shouldn't need any preamp at all for your close stations. Maybe a separate antenna with no booster pointed in that direction would work best for them, then you could use some other system for the distant channels that are breaking-up. You might even get away with just an inside antenna for the close stations. A dual antenna system can be combined with a "jointenna" device that only takes the intended signals from each antenna reducing the possibility of interference from multiple receiving devices.
I'm not familiar with the booster you have. The gain is only one important factor in the performance equation. You should also have a very low noise figure (like less than 3dB) and that's not always the case with high-gain models. Also, where you have a mix of both close and distant stations you probably need one with a high overload margin, like the Winegard HCP-269. It only has a moderate gain (~12dB, IIRC) but the highest overload factor of the consumer-grade models, and a really good noise figure as well.
I'm not sure about your booster type, but in most cases the preamp part is contained strictly in the housing that's mounted at the antenna. The other "half" is just the power supply. (A notable exception is some of the Blonder-Tongue units that also have some of the active RF circuitry in the inside unit.) You don't want to mount any power supply outside. There's generally no advantage to mounting it any closer to the outside unit if your total cable run is 100' or less.
Hope some of this helps...