Ana-
I've had the same problems (and more) with my John Deere lawn equipment. My father warned me that there smaller equipment is crap, and I should've listened.
I've been thinking about sending them a letter (public relations) about how they are shooting themselves in the foot. Since then, I've bought two farm tractors (both New Holland), two balers (New Holland and IH), a backhoe (Case), and two bulldozers (Cat). I avoided John Deere because of the problems I had on their lawn equipment, and it really hurt them when I went bigger! I talked with my wallet, which will eventually make them take notice.
I bought one of their SST lawnmowers, and had two gas tanks fail at the seam. I had an electronic problem that took forever to find, and problems with flimsy cowling being in places that is sure to get cracked or broken off when mowing close to trees, etc.
Some "customer satisfaction" rep. was unlucky enough to call me during the middle of all my problems, and I gave them about 30 minutes worth of my story. I offered to sell them back the mower for 70% of what I had paid for it 4 months prior, and they acted like I was insane. They sure didn't want it back though! And, of course, nothing ever came of my complaints....
For heavy mowing, I've heard the Toro Commercial stuff is good. For heavy duty weedeating, Shindawa is awesome!
Sorry we had to learn the hard way!