The suggestion from symfrog to check for intake air leaks is on point, leaks were fairly common on the older Twin Cams especially at the manifold to head connections. Other possibilities include leaking injectors, a TPS (throttle position sensor) reading error, or of course the IAC (idle air control) or it's wiring.
First check for diagnostic trouble codes. There are instructions posted on this site if you are not already familiar with how to check for codes. Checking for intake air leaks and leaking injectors are simple processes that require no special tools, testing the electrical items may require special tools.
To check for leaking fuel injectors, start by removing the air cleaner and blocking the throttle plate wide open. Then turn the ignition switch on for two seconds, then off for two seconds, and repeat that process about five times. Visually inspect with a small flashlight for the presence of raw fuel in the throttle body and manifold bores, there should be none.
Jerry