What I was told by the operator at the RR day where my fuelling issue was discovered was that the fuel/air ratio is meant to richen up as the exhaust gas temps increase, typically due to higher engine RPM. This means that it'll start about 14.7 to 1 from low engine speed and should drop steadily towards about 12.5 to 1 near the redline. 14.7 = 1.00 in the lambda figures, 12.5 = 0.85. Yours is doing that, just it's jumping about a bit in the process. Here's my fuelling graphs:
Ignoring the very high figures at the start, those were when the throttle was closed just before starting the run. You can see that the numbers stay pretty well bang on 14, they're suppose to curve downwards to about 12.5 by the end of the run.
In your case it could be a faulty fuel pump, fuel filter, fuel pressure regulator or possibly the vacuum hose going to the regulator. If you remove the engine cover you'll see the fuel rail sitting just on top of the intake manifold. At the passenger-side end of that there's a small round thing which will have a black (probably) hose coming off it, that's the regulator. Worth checking that the hose is in good nick and secured properly.
Here's my thread if you want to read up on what's going on with mine so far -
http://www.seatcupra.net/forums/showthread.php?t=282854 - I'm taking it to a garage to get fuel filter changed and injector seals replaced (among other things, due a service anyway). I'll post back in the above thread if there's any news.