My fiancée's car went into limp mode with the above error message yesterday morning on her way to work.

I got a the code reader from work and it had the CEM-U040086. Cleared it, took the car a run, no issues happy days.
The misses then took the car to sewing lessons over the town and I told her to drive easy and expect it to go into limp mode again because I hadn't actually fixed anything and was running a trial and error type thing.

She got there (just over 5 mins drive), no issues at all.
After a couple of hours at her class she was pulling out of the car park and she got the turtle message again and into limp mode. When she got home, I plugged in the code reader, no error messages which was odd.
After trying to figure out what she was doing when it went inro limp mode on her, all I've been able to find out from her was she was accelerating trying to get up to 40mph. After further questioning she said "I was only about 3500rpm trying to get up to speed"..... I'm saying nothing...

Anyway, before work this morning I took the car for a run up the road.
Runs perfect up to 30mph, from there up to at least 60 there's greatly reduced power, kind of jerks up the speed. Happens in every gear the cars under load (going down hill it seems ok?).
I had no warning lights on the dash, no Turtle message, and no error codes when plugged into the work computer...

Anyone any idea in the direction to start diagnosing? If I even had one error code it would of helped.

Thanks in advance.