94 SC2 "Service Engine Soon"

I have an automatic 94 SC2 with around 150k(stopped at 101k) and the "Service Engine Soon" light just came on...It has always had a few issues, but the light seems to come on after you drive for around 15 min or so and then stays on until you shut off the car. It has always idled high-1100 in drive and then jumps up to 2000 if you put it into neutral, but it seems to idle higher after the light comes on. It also seems to shift up and down quickly when it downshifts. Any ideas? Two different problems maybe? Thanks for any suggestions!

If you could find out what codes are causing the light to come on that would help a lot.

ok...the Service Engine Soon light was cause by a hose that came loose....fixed it!!!! Any idea on the tranny?