The early 3.0L Roadsters were traditional cable to throttle body, if yours is, suggest you check the return spring & exposed cable end are not gummed up with sticky dust, clean up and re-lube and try and get a bit to track down inside the cable.
The rev hang you describe is not uncommon on Roadsters, even the 3.0's, you can simulate this to test...
Handbrake on hard, 2nd gear, bit of revs, tickle the clutch to bite, hold, let the revs off & because the engine thinks it's under load & may stall it holds the revs even when you dip the clutch (it's a sort of anti stall) I used to be able to get mine to do it at tickover creeping out of the garage and up the driveway incline
