Surge problem at speed

The things changed were the distributor and the control unit.
Now its trailor hitching.
New distributor probably has very different curve and characterists.
Or they gave it too much initial.
Now its trailer hitching. Yes it could be too much timing in certain rpm and load conditions. Since the conditions are light load vac advance is a part of that.

Replace the magnetic pickup in the old one and try that, and or try the points distributor.

The only way to know what the timing looks like in an aftermarket distributor will to measure the timing at various rpms and then plot it.