Sputters but so far has not stalled. Dealer has checked spark plugs.
Because it is intermittent, doesn't always occur while being test driven.
Engine light has not come on. Feels like it is about to stall.
Check the intake air snorkle and air filter "box" for disconnected, broken or cracked hoses.
You can also use carb/throttle body cleaner to clean the Idle Air Control (IAC) solenoid and throttle body.
Another thing to check is the PCV hose. Often these hose will get a hold in them (from age and engine heat) and cause rough idle and/or stalling. The PCV hose runs under the intake manifold down to the engine block. It is kind of hard to see and reach so do this when the engine is cool.