Overnight onset of lag with shifting

[Gerrychuck]

I have a 2008 135i M Sport (N54) with a Wagner FMIC and a Cobb AP 1+ tune. Changed plugs last year, water pump and thermostat changed a couple of years back, and walnut blasting done last year.

Car has been running beautifully all season. Hadn't driven it for a few days, but took it out today; ran fine when accelerating moderately during warmup, and had normal minimal lag when giving the throttle a good squeeze in any given gear (i.e. running at 50 kph in 3rd, give it some throttle, immediate torque rush). Also seemed to build boost normally leaving a stop in 1st. However, huge lag in response after every upshift.

I will probably reflash to stock to see what that does, and perhaps reinstall one of the more advanced tunes after if that helps, but any other thoughts on what might be going on here? Also, as a note, the water pump seemed very active, which concerns me as that's what it did before it failed, but the new oem pump only has a few thousand km on it, so it seems unlikely to be screwed again. Will check for codes too, although no CEL.

[Gerrychuck]

Update: Took the car out again this afternoon, and the car ran perfectly. No lag, no unusual water pump noise. When I drove it this morning, I started from cold, drove a short distance, shut down for 10 minutes, restarted, and experienced the symptoms while taking a long way home to make sure the car ran long enough to get more thoroughly warmed up. I've had it throw a couple of sensor codes under those circumstances before; I suspect that's what happened today, even though I didn't get any fault indications. In any case, I think I'll leave the tune along for now.