I have a 93 Taurus 3L. After driving at 65 mph for over one hour, the idle will not kick down. Any ideas on the problem ?

My guesses would be, in no particular order, vacuum leak, IAC valve sticky or malfunctioning, or possibly a throttle cable that's sticking. Is the check engine light on? What's the codes?

The check engine light has not come on so there wouldn't be any codes to get. The throttle cable is not sticking. The IAC reads 12 ohms which is between accepted ranges. I'll check for vacuum leaks. Thank you.