FYI... A quick "image search of inline electric fuel pumps revealed that look-alike pumps as yours pictured, indicate "4-7 PSI 34gal/hr" (and some even higher up to 10PSI). That may be a good indicator of "too much" fuel-flow and PSI "designed-capability" to be consumed at low speed/volume requirements. That could also explain the "extra throttle rigging" to allow more air??? Maybe not.
I did find this pump listed ...
https://www.ebay.com/itm/42S-Low-Pressure-2-3-5PSI-Universal-Micro-Electric-Fuel-Pump-Polymer-Gasoline-/265814190393 ... but even that one exceeds/doubles the Solex carb "needs" as stated to be at just "1 PSI". Could you be drowning the poor thing at low speeds, and also overheating the pump that can not get to produce the "designed flow" (aka cooling) from "blockage", therefore heating the over-pressured fuel as well??? The pump may be overheated and quits until "cooled off".
How ironic, my other post was against "guessing", and here I am "guessing like crazy". Sorry, but I do feel like your electric pump is not your "best situation" from your descriptions of "symptoms". I did see the Holley fuel regulator of Ron's offering verifying the "1-4 PSI" capability, but you may also need a "fuel pressure gauge", again hard to find one down to "1 PSI indicated", to regulate the pressure correctly and accurately.
I hope the above info helps target "next steps" for you to get back "on the road again" happily and reliably. Good Luck, and please keep us posted of your adventure.