2001 Honda Civic Mystery problem

I've had this car for a few months now (I bought it from my sister) and everything was fine until a few weeks ago when "it started getting hiccups". The car would skip when I had the cruise control on on the highway but I thought, hey, it's still running and there's no light on, so it must be fine. Well last week I was driving it in town after school and the car died and all the lights came on and the steering was basically impossible to do. The car wouldn't go over 20 when I turned it back on, so I went and stopped at a gas station and turned it off for about 15 minutes. After that, the car turned on and was fine. It was going well over 70 and got me back home (15 miles from town) The next day we took it in to get fixed, but the mechanic found nothing wrong with it, and just replaced the spark plugs and gave it back, telling me it was fixed. I drove it around yesterday, first on the highway, then through town, and 30 minutes into the trip. The car stalled, wouldn't go over 30 and had all the same problems. My car is now sitting at the shop with a mystery problem. Please help!