I like your car mate! I would deffenitly recomend the upgrade of the starter to a ford Transit starter (see other threads). It's so shit to wait till you can fire it up again when its hot!
However I have quite a simulair problem like yours... It has been in my car for a few years and couldn't find it till now (i drive it only in summer and haven't been spending time on finding it)
So my problem is the car won't pull to the red line all the time. I am pretty sure its retarding the ignition. It starts sometimes around 5500rpm and when i shift to the next gear and floor it then its there. Engine sounds strange. Does not build boost, but it does keep boost. I lift hit it again and off the car goes.
So i am pretty sure it can't be the boost lines. Now something happend that made me want to investigate this problem again.
I was driving. The car slowely got warmer. I noticed it while i was almost home. I set the ventilator fully on and heater on warm. The car did not cool but stayed a bit high (not in red). The same beheavior i have on high rpm's and after shifting started to come up earlier and more frequent.
When i arrived home i noticed that there was no coolant in the header tank and i had a leak on the back of the engine block. So i added a slight bit of coolant (not too much as i didn't want to thermoshock it) and let that pump around and then switched off.
What can all retard ignition? For me it seems to be temperature related? Temperature on the dashboard is fine, the thermostat is not too old, fan kicks in at the right time. What other sensor is there that the ECU might use?
Do you only have it in 3rd gear? You can keep your foot on the pedal and then it goes away? You say it starts in low rpm after hitting it, do you mean low rpm in the next gear?
One of the quick checks is disconnect the gear connector on the right side of the engine. It's the knock sensor. If that sends a faulty signal to the engine it might retard. If you pull it off it sends no signal, so no retarding for this sensor. But becarefull if you do knock then it might go worse then before!