Throttle cable ?

My car at cold idle is perfect. At hot idle it's perfect. My problem is when cruising and I put the clutch in to shift the rpms stick for a couple of seconds then go down. If I take my foot of the gas in any gear it's sticks at that rpm for a couple of seconds then goes to normal. I know about the idle checklist but the car idles great. Could this still be a vacum leak, IAC. EGR or throttle cable ? I'm going out today to start checking it out and thought I might as well ask to see if anyone else ever heard of this happening.

88 5.0 supercharged 8#'s 5 speed

I will check for the spring when I check the cable. I don't think it has anything to do with the boost because I had all that done with a dyno tune 2 years ago and this just started happening at the end of last year and the first couple of times I had it out this year. I'll be going out there in a while and I'll start with the cable. Thank's guys.