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engine hesitation under full load acceleration

Engine

Vehicle Information: Year: 1986; Engine (Size, Modifications): 930 3.3L US spec stock; Total Mileage: 148414;

Recently developed unusual symptom - under full load acceleration starting at around 4000 to 4500 rpm, occasionally 'stumbles' or hesitates - checked fuel delivery, CIS pressure settings, replaced old front fuel pump (drawing over current but meeting delivery requirements), pulled/checked plugs (W3DPO as spec'd) all burning good, checked wires all good, replaced cap/rotor, no change. No detonation like fuel starving, and have installed air/fuel ratio meter - mix stays right on 13:1 with no drops. Have mechanical boost gauge in clock position, no flutter or waver in boost pressure. Does this at any temp range from engine just slightly warm to full hot, any range of ambient air temp. Had several tanks of fuel through it, no difference from tank to tank. All gears. No tach flutter like ignition dropout. Any ideas where to look next, or what this might be??

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