I posted a similar question recently where my 2004 dispatch van was stuttering slightly at motorway speeds when maintaining a speed - basically, when doing 60ish mph, it would suddenly feel like the van was hit by a head wind. A slight press on the throttle would solve the issue.
Since this, I've replaced both the fuel and air filter, which made no difference. I've also taken it to a garage who plugged it into the diagnostic machine and no faults were showing.
I have used it earlier and it was doing the same, but all of a sudden, it started jerking when trying to maintain a speed, as if the engine was trying to stall. This would stop I I floored the throttle or got off it completely.
Basically, I got home driving it like a 2 stroke, throttle on full, or throttle off.
Temp is normal, oil level ok, water level ok, no strange or unusual sounds. No engine management light. When the throttle is fully down, it just accelerates normally with no issues. When I'm still in neutral, I can accelerate through the revs with no issue or stuttering, so it's only when the engine is under load.
Last time I posted, people suggested air flow meter or throttle position sensor. I haven't changed these as I thought any fault on these would be shown on the diagnostic.