If, as you seem to be indicating, you can induce surging by leaning the Brick onto its side stand, that is BIG news. Replication is a key element of proving an hypothesis. All you need to do is consider which elements could be affected by gravity, or G-forces (as in the case of occurrence at speed). Pick one after another—wiring, hoses, switches, for example—and wiggle or move them attempting to cause an effect while the engine is running in neutral on its center stand.
The pedestrian element of a defective fuel pressure regulator should be resolved. Verification of work claimed to have been done, such as valve clearance measurement, should be undertaken. Many owners seem to have been disappointed by BMW service centers.
Obstruction in the air snorkel should be investigated. Another element to consider is the air flow meter. Is there obstruction in its intake; is its air temperature sensor dirty—there is a dedicated cleaner for that; is its electrical connection compromised, does the movement of its internal vane appear to drag or is the movement unregulated?
Diagnosing an ailing Brick should be an orderly process of system-by-system investigation. Preconceptions and assumptions made without sufficient depth of experience can stall resolution. It can be tedious but it should be focused. Appearances can be deceiving.
