I got a weird one. I'm fairly car literate and I'm usually able to pull in to my local independent shop and tell them this and this need to be fixed and replaced. But this has me stumped.

I have a 2016 Q5 2.0 engine that went bad with carbon deposits yada yada... So I stepped up got a new motor from Power Engines it came all crated up they were super helpful pleased with them. My mechanic moved all the bits and reattached them to the new engine from the old. Everything had been working just fine on the old engine up until I put it in storage for close to two years due to a compression issue (Carbon ring deposit blow by)

Well here is the problem granted this info is incomplete and secondhand. The mechanic gave me the keys I drove it ten, to fifteen miles and had to have it towed back. The whole time it had a skip a tumble in power it hated slow acceleration between 1600-2000 rpm kind of missing and not solid power. Well the dang High pressure fuel pump apparently broke loose. Mechanic thinks it was pumping air and not fuel and that the low pressure fuel pump wasn't providing enough fuel. He ended up replacing the High pressure twice (for a total of three the original plus two replacements) then the low pressure fuel pump once and spark plugs. He said twice the high pressure fuel pump blew, and the low pressure was fine but replaced.

Well I got the keys back and something isn't right there is still a power tumble similar to the original problem, but the weirdest thing is I can floor it and it will not speed up. It does it between 40-50mph once it got stuck at 40 twice another around 46 or 50mph five times I've seen this but it's not always. If I usually back off and reengage the pedal it will then gain speed but just so weird. What am I missing what did my mechanic do wrong..?? Where do I start in trying to diagnose..??