4,800rpm to 5,200rpm is peak load so if you have any fuelling or timing issues this is where detonation is most likely going to take place. Therefore the ECU may of pulled timing to protect things.
The same load effect can highlight plug issues, so I would start with a fresh set of plugs, followed by monitoring the boost(temporary plumb in a £20 boost gauge).
If that all looks OK and no improvement is seen then you really need to get the fuelling checked. The only way to do this is either on the road with a wideband probe up the exhaust or on a dyno.
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