As Keithy suggests, the most common causes of erratic idle speed are due to vacuum leaks allowing un-metered air into the engine. Particularly check the intake hose between throttle body and cylinder head; there have been one or two cases of premature perishing and cracking of this hose. Also examine closely the plumbing on the fast idle mechanism on the bottom of the throttle body. Under load, or on a wider throttle opening you aren't going to notice anything untoward as there is little or no vacuum in the inlet manifold. Vacuum is greatest at idle with a closed or almost closed throttle.

Other things are possibly... A frayed throttle cable inner (on either cable) which can go unnoticed and cause the throttle not to return to the stop reliably or consistently. Incorrect valve clearances, particularly tight valves, can also give rise to odd idle speeds especially when the engine is hot. When were they last checked?

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