Well, I won't say with 100% certainty (only death and taxes are that certain
) that you'll get better reception, but you are VERY likely to get good reception with an outdoor mounted antenna.
If you mount your antenna 10' above your roofline, that gives you more elevation to receive, as well as nothing impeding reception, such as trees, other houses, etc (assuming you aren't in some very low lying area).
What you might also want to try is a good low-noise preamp with your attic mounted antenna. That might give you enough gain to alleviate the dropouts and stuff.
You might start with a 10dB amp from Fry's. It's in-line (you mount one part right at the antenna, and the other at the HD reciever). The item looks a bit cheasy, but it does work. Not necessarily the best amp in the world, but it might be all you need. Otherwise, I'd recommend a more "professional" model such as the channel master 7777 preamp. It has 26dB of gain. It allowed me to get Fox at my house (in Scofield Farms - about 8 miles from the towers) with a CM4228 antenna, which I have since gotten rid of b/c CBS upped power and I don't really care about fox at this point.
Adding an amp may very well help the digital stations, but may hurt any analog reception your receiving with that antenna -- for example, ghosting and multipath may get worse.
Ok... I'm rambling... just ask more questions if you need any more help. Some of this HD reception thing is just trial and error, especially while some stations are low power and some are high power, etc.