1975 MG Midget Sputtering engine when warm

I'm the 3rd owner of a 1975 MG Midget. She starts great and runs fine until the engine get's real warm. The problem starts (i.E. The engine sputters like it's not getting enough gas or has the improper mixture) after an hours driving (at 55 MPH). This problem also happens if I fill tank up with the engine warm.
I can usually drive through the problem but she almost conks out.
I have tried the following fixes to no avail. I live way out in the bonnies of the N.E Georgia Mountains. No British car mechanics within 90 miles.
1) Checked fuel pump pressure. It was strong.
2) Checked fuel lines (there was no dirt/debris in line).
3) Carburetor was rebuilt one year ago. (I didn't drive much last year. The problem only started recently).
4) I was advised by a MG friend it could be a possible fuel boil issue. It was recommended I put 3 or 4 wooden clothes pins on fuel line to disperse heat. No luck.
5) I replaced fuel filter. No change.
6) I was told there a pressure choke in the Stomberg carburetor that be malfunctioning. The local shade tree mechanic says it is working fine.
7) I use only the highest octane gas.
8) Plugs were recently replaced.
9) A for a fee on-line mechanic says because the emission control system was disconnected by a prior owner that the disconnection was the problem.
The local mechanic sees no issues with the disconnect.
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Please do you have any ideas/suggestions?