Questions after a few recent US Airways flights

During a honeymoon fiasco (that was our fault, passport troubles on a flight to Cancun Mexico) we ended up taking six different US Airways flights. As strange as it sounds I have to say their customer service was nothing short of amazing which will come as a shocker to most I'm sure.

Anyways, I noticed on all six flights that the plane would taxi on one single engine. The pilot would fire up the other engine shortly before takeoff. I also noticed on each of the flights that once we were in the air and approaching cruising altitude that the plane would slow very rapidly all of the sudden and then fly normal. The rest of the flight you could hear a very low-frequency humming and vibration noise that sounded exactly like an engine at idle. As we began landing procedures you'd hear that engine come back to full steam. Each time the engine went back to full steam you would see a flicker of lights and the inside air would kick off for a few moments exactly like it does when they're firing an engine up on the ground.

Is it normal to cruise essentially on one engine under full power and the other idling? We flew on A319's and A320's the entire trip so it was different planes and they were still doing it. If that wasn't the case, any ideas what it might've been?

BTW, A319's have to be the most uncomfortable plane I've ever flown on. The seats are garbage! A320's have the most annoying-sounding turbine engines at full-song I've ever heard as well. Sheesh. It would get into your head and about drive you nuts the entire time they were taking off. I couldn't stand it! I missed flying on 737's the entire time that's for sure!