I am obviously new here and have a question for those more knowledgeable. I am thinking about buying a new bike for myself but I am having trouble narrowing down all the different types, styles, and classifications. I am hoping someone can help me figure out what type of bike to look at.

I bought a Specialized Expedition back in 2001. I was told that this was a "Comfort Bike" and I liked it very much. At that time I would ride to the train station (about 2 miles) a couple of times a week and take 5-10 mile rides on the weekends. That bike was stolen only 2 years later and I replaced it with the same model in 2003. I have been very happy with it until recently.

I now ride between 25-35 miles at a time about 3 or 4 times a week. I ride mostly on the street and on paved bike trails with an occasional sprint on dirt or grass (because the roads and bike trails don't always meet). I am in my 40's and I ride for fitness and because I really enjoy it.

I am having trouble with my current bike now that I have upped my distance (and pace) and so I think it is time to switch. The problems I am having are:
1) When I need to start pedaling harder (up a hill for instance) the bike creaks a lot and I assume is the frame flexing
2) While I like sitting up (I have back problems sometimes) there are times when I am riding into a headwind that I really wish I could get into a more aerodynamic position. Headwinds suck.
3) My bike is really heavy. It is probably about 35 lbs.
4) Even after getting a tune up, the bike seems to be a bit sluggish
5) It is really difficult to maintain a pace of over 15 mph - guys who don't look to be an any better shape than me blow by me on the road all the time. (I would rather blame the bike than myself)

The things I like about my bike are:
1) It is very comfortable to ride
2) It doesn't hurt my back
3) It goes anywhere I need it to.
4) It is really stable going up or downhill at any speed.

Now I don't know what to look at. Should I be looking at a hybrid or a road bike or cyclocross or what? There are so many classifications that I find the whole thing quite confusing.

I would appreciate any help I can get in narrowing down the field.