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turbo stumble

Engine

Model: 944 Turbo, Year:1987, Mileage:85000, Type of use:Street use only
I have checked all the information on "turbo stumble" in the Tech Questions. My problem is under hard acceleration in 3rd - 4th gear under boost starting around 3500 to 4000 rpm. Note the car will pull hard under full boost with no problems in 1st and 2nd gear up to the rev limiter (which I have found by mistake). The engine will rapidly lose power in very short bursts almost like it is cutting on and off rapidly. It will give a backfire type pop in the exhaust. Is this the waste gate opening and closing? My boost gauge shows almost 2 bars. I assume this reading is due to instrument inaccuracies. I have replaced the cycling valve and all the rubber under the hood. I have pulled the turbo to replace oil seals so all of the exhaust o-rings are replaced. I ran through all of the DME/KLR Troubleshooting (except O-Scope and blink codes) and had the DME repaired (bad capacitors) and the KLR reworked. I have also done a complete tune up. The only ignition part not replaced is the coil. Any suggestions?

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