Have a 95 Pontiac Transport with an idle problem?

[Bob]

What it does is idle up and down after warming up a bit. I got codes p0341, p1640, & p1670. The p0341 code is for the crank sensor performance. I just change sensor after I got the codes but it is still doing the same thing. The other 2 codes I'm not sure what they are and what I'm to do to fix all of them. It has a 3.8 motor. Can anyone help me out with this I do not want to take it to a shop I would like to try fixing it myself. Thanks