Does the engine have a throttle cable/linkage or is it drive-by-wire (ECU controlled) ?
If it's drive-by-wire, you're screwed; as the computer is dropping the revs slowly for emission reasons. (My sister's Saturn VUE manual tranny did that and it drove me nuts....)
It is drive-by-wire, and you are right that this is what's keeping the RPM up. It would probably require a software revision to "correct" that.
