Engine is sputtering upon acceleration. What may cause this? on 1991 Volvo 940

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Injectors & throttle body have been cleaned. Plugs & plug wires have been changed. Gas & Air filters have been changed. Water pump, Timing belt have been changed. Turbo has been serviced.
Car starts and idles well once cold. However as engine gets warm and begins to be revved at 2000rpm sputtering occurs and it drops off to 1000rpm and after a short time delay corrects itself.
(1) Answer
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