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Hi Looking for ideas on how to diagnose an annoying/intermittent problem. 1993 Subaru Legacy RS EJ20TT, 198k on the clock. Rarely but atleast once in the weekend - At part throttle the car stumbles like an engine or fuel cut but comes back immediately. Usually only does it once or twice. No issues under full throttle, e.g. clear on ramp powering up through the gears and boosting. Can drive around all day and not have it happen. Seems more prone after the care has been in use for a number of hours. Or being reused later in the day. Car idle is stable considering its age. Bit of fluctuation/hunting but all within about +/- 200rpm, aka normal for the car. When: #1 Most often occurs in 4/5 the gear driving along at constant speed around 2000 rpm. Hickup and continues on. Both motorway and clear city roads. #2 Had it once going up the driveway in 2nd gear around 2-2500 rpm. In this instance rpm dropped away (suspect because of the hill) before coming alive and continuing up the drive. #3 Happens now and again when gently pulling out of side roads around the 2500 to 3000 rpm. From the gauges - don't see any apparent drop in RPM - don't see any manifold pressure (vacuum) changes No check engine (CEL) light or codes when using the 1 wire plugs. Things done to date: - replaced spark plugs (over 100k km on them, but looking clean) - compression tested - ok based upon cold cylinders - changed crank and cam shaft sensors, just precaution - cleaned MAF, multiple times with little effect - intercooler and piping has been off and on multiple times with no impact or obvious pipe splits. - swapped ECUs, back to factory from a MINEs one. Still happens although slightly less frequency. - upper throttle and idle air control valve foam clean, no impact - changed a number of vacuum lines that were brittle, no impact - cleaned and flushed the vacuum assemble for the twin turbo circuit. No obvious leaks or switch issues. Other factors - been having issues with water rushing the in the heater core. Replaced the core as got blocked with stop leak (prevention). - no detectable coolant leak and a pressure test (when cold) holds for a while. NO overheating ever. - Even if there was a coolant, can't see how it would cause a intermittent stumble. Would expect poor idle. Thoughts? - fuel pump, fuel pump relay, injectors, regulator - I would expect these to be worse under load if an issue - sensor, electrical - can't think of anything else that would impact at part throttle and not throw a fault code. ANY Ideas appreciated Thanks Raymond