Intermitted loss of throttle response - 2009 Harley Davidson FLHTCU Ultra Classic Electra Glide

I would want to see fuel flow and pressure numbers.

sounds like your throttle position sensor is having problems some symptoms are uneven or rough idling, intermittent acceleration, no response at all.

A long list of possible causes including low fuel pressure and a faulty throttle position sensor.

your unstated OS is infected
there are only a few choices
dis-infect it, (long story, and endless examples if you google that)
reload, the unstated OS
or up grade the OS

is this XP?

Have the alternator checked to make sure it is producing 13.65 volts,to 14.25 volts at the battery under a load,headlights on,and ac on high blower motor.

That could be a fuel system problem like a fuel pump or a throttle position sensor

your throttle pot needs replacing. your thumb lever attaches to the throttle pot.

you still have air in the system,you need to cross bleed the system,RR,LF,LR,RF.if this does not work you might try picking up a hand operated vacum pump to help get all the air out.

something deeper... timing is not it... plugs maybe... BUT, you need to pull the plugs and see what they look like,,hopefully they will look normal...if they have strange looking deposits on them... time for a qualified professional to look at them..(leave them as they are...do not clean them)... and your bike...
.. the throttle cable may have been pinched by the steering head stops that are welded to the frame...(this is not an uncommon occurrence :) ).. the throttle cable will have a spot that is kinda "kinked" or have a knot looking spot in it...replace it...

ignition switch on its way out, as a golf owner im sure awre theyre common, also possible ignition relay, though i reckon ignition switch it has two outputs and they fail intermittanly, eventualy they will stop all tgether,new swith bout £20 though fitting means removal of steering wheel and a puller is required to remove the housing.
Good luck Davelee