At a steady 3000rpm (60mph in 4th) I can feel the engine stuttering/hesitating.
Any ideas on what might be switching at that rpm?
Engine is fine either side of that point.

Car is a 99MY UK turbo with PPP ECU.

 

what fuel you running it on?

 

Flat spot in map?

 

99/00 are very prone to following the lambda sensor.

Could try unplugging it to confirm.

Is it worse when the engine is cold?

 

"Following the lambda sensor" - sorry, I don't understand what that means?

There are no fault codes for a lambda, although it is about 10 years old (and a cheapo Halfords part...)
The exhaust is a straight through de-cat system.
It is indeed worse when cold (I think).
You could see the injector timing jumping back and forth on the PSI3 monitor (when it worked) but there is definitely something switching at 3000rpm.

Thanks for the help!

 

The ecu adjusts the fuelling to he lambda sensor reading and you can feel them adjust often on a 99