Just had an MSD do the same thing Friday on the dyno--swapped out the distributor for fresh MSD unit and whamo! All good and pulled like it should-- It can be the module in the dist as well--you would never think it--I thought it should work or not work but found out different Engine would just layover above 5000--put exactly same curve we always use black stop and one light silver one blue spring--and she went to work and ran great--always...I mean always something.
I bet that Hemi is running out of fuel--has an elbow or something in the system or just not getting full voltage to the fuel pump--we always see giant swimming pool filler pumps with NO relays etc check and they are just not getting voltage--not pumping "what the box says" A drag car can run good with no symptom other than slow and low MPH--get fuel straight and it can really make a big diff
China regulators are trouble too--Best for the $$ is Aeromotive stuff--class act quality and good support--regulators are very nice units but remember are o ring fittings not pipe thread when planning your plumbing.