95 Plymouth Neon.A lot of difficulties lately.

O k...for a while now my neon has been sputtering when I stop and then go...then when I am driving the RPMs will drop...it would idle high...and yesterday the check engine light came on, but then went off after a little while. I took it to Midas bc they were the only ones around that were open on a Saturday and didn't makeme wait all day...they changed the spark plugs bc I was told that is what was wrong with it...it isn't idling high anymore and it sounds better but last night on my way home from the store the RPMs started to drop again. Do you know what it could be? I had bought the car the past Febuary. I don't make a lot of money so it has been hard trying to get it fixed. I replaced the tire rods on it, the front struts, a few hoses that went to the engine, motor mounts, transmission mounts and a seal on the trans axel. I know it needs a lot of work. But what could be causing it to do what it is doing?