What would cause my truck to ramp up on RPMs and then shifting hard while correcting itself?

I have a 1992 Ford F-150 XLT 5.0
with an automatic trans. I bought the
truck while it had been sitting for
about a year, and didn't run. After
replacing the battery, starter, starter
cylinoid, brake lines, spark plugs,
one coil pack, and some other things
it ran fine. However, now while the
engine is idling it idles hard
sometimes, and often dies while
reversing from park after it has been
driven. The truck will stay the same
speed while the RPMs are spiking
upward, and it shifts hard trying to
correct itself. The check engine light
with come on, and then go off after it
corrects itself. What could be the
problem? Thanks for reading, and let
me know if you need any further
knowledge because I'm sure I didn't
cover it all. Thanks in advance!