750 iL won't accelerate

I have a 1990 750 iL and I don't exactly know whats going on with it. I'll start her up and as she idles she makes a low grumbling noise. Then, the check engine light comes on. When I try to press on the accelerator, the car moves, but accelerates very slow.

The gas pedal will be on the floor and 10 minutes later, i'm only going 40 miles an hour. It used to do this before, but I would shutdown and turn her back on and she'd be fine and wouldn't do it again for another couple weeks. Now, its always like this every time I turn it on.

Can anyone help me with this problem? It's rather frustrating.

If your model has the manual/automatic switch (I don't know if your particular one does), check to make sure you haven't set it to manual by accident.