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(after around 4k rpm). My suspicion points to the ignition system.+ rotor less look good,(now filled with silicone)(original, yet to test)(Not knock sensor or fuel cut)(1500 rpm). Could it be possible it's slipped a tooth on timing or would the coil and ignitor be a good items to replace first?? are you running higher boost? What fuel are you running? I have been driving in a lot of rain. The spark plugs look tired (after 6 months) as well.(aftermarket) vac hose split where it was "T-pieced" into the system and I had the same outcome as you are getting. Its been going on for about 9 months so the car has been wet / cold / dry / hot and it doesnt seem to make a difference. My first thoughts was the cold and damp last November, but Ive since ruled out the damp as the car hasnt been wet expect for washing . Also, Ive tried it on a variety of fuels........& King leads , cambelt and pump changed. This made no difference what so ever. / clamps just incase. No difference. No difference./ igniter pack - woohoo! Back to normal 
This is how my 2 step rev limiter works. Destabilizing the coil by grounding the negative signal wire.(in case of boost leak),(slight coating of oil),(If it was indeed damp inside). The old one is 22 years old and could do with changing. It could have got damp and caused the issue (as it has a crack). Matt, good luck with the fault. I would double check all of the wiring to each sensor, earthing points etc.