If you're doing a partial boil with extract, I think leaving all that sludge at the bottom may be what's making your gravities so inconsistent. With a full-boil, you can leave some sludge at the bottom and it won't affect you're OG. With partial mash, you're leaving some very sugar-rich runnings at the bottom. One Quart out of a 2 gallon boil is like 3/4 of a gallon of a 6 gallon recipe (my usual size), and that's assuming it's homogenous (which isn't always true).
Perhaps consider putting everything in the kettle into the carboy and diluting to an appropriate volume. Then wait for all the break/hop sludge to settle (couple hours maybe) then rack to a second carboy before pitching the yeast. If you have good sanitary practice, there shouldn't be a serious risk of contamination (though I wouldn't if you worry about your sanitation).
Of course the best option would be to go to a full boil. You can get a 7-8 gallon pot for ~50 bucks for a stamped stainless steel kettle. Less for aluminum. It's basically the cost of one batch of beer (or ~12 of the extra pounds of DME that you're putting into your beers already)
You may also wish to consider switching to 5.5 gallon or 6 gallon recipes, so that you can still end up with 5 gallons after leaving sludge behind at each step.


