2001 Hyundai Accent Strange problems.

First, my check engline light is normally lit. I have a cheap obd-2 scanner that only gives the letter E for the error code instead of the 4-digit number.
Here are the symptoms my car is having:
1. When stopped at a stop sign or a red light, if I do not put it in park or neutral, the engine will make strange "chugging"-like sounds and get a little "jumpy"
2. If I accelerate too rapidly from park, especially up a hill, those noises will be louder than normal, it will get more jumpy, and my check engline light will blink for a few seconds until I slow down to a crawl basically.
3. It also seems like when the tachometer is over 2,500 rpm's while I am driving it makes those chugging sounds whether I am going uphill or not.
4. Backing down the driveway in reverse makes the check engline light blink with the same bad chugging noises unless I go down really fast, or coast down in neutral.

Within the last 1-2 months, I've replaced the spark plugs and used a fuel additive that is supposed to clean the fuel injectors and that sort of thing.
I change my own oil and oil filter. It seems like this started happening last time I changed it, but as far as I can tell I never did anything wrong.

It gives me the feeling that it's going to conk out on me at a red light one of these days. It doesn't matter what the weather is like or anything.

Any suggestions as to what this could be? Thanks