E30 help: bogging when warm under load

Hello! I’m helping me nephew with his first “GRM” style car purchase: a clean 325i sedan.

the car was running great at purchase. We’ve done the timing belt, ignition parts (all) and water pump. 

After a week or two of driving, it started to bog under load above about 4K rpm. It seems typical behaviour is that at warm restart it runs very well, but after a few minutes starts to run out of steam at higher rpms. It can maintain speed just fun.

it feels like fuel, but has a recent pump replacement in its history. Transfer pump appears fine.

AFM is moving freely.

Any other areas I should be looking at?

I would be looking at fuel pressure. New pumps can go bad. I would be looking at the FPR. It could be as simple as a fuel filter.

How is the hose between the AFM and throttle body? Any cracks?

AFM moving freely means next to nothing on an E30.  Hook it up to a multimeter and check its resistance while moving the flap.  If you see any blips at all, replace it.  If it isn't perfectly linear up to WOT, replace it.