No ECU 'jiggling' would be needed (possible?).
I suggested to the previous owner that he swapped the throttle body back to original, so he may already have tried that. Ask him.
I know he had investigated the throttle-stop as causing a problem and that didn't help.

There is masses of technical information here for full-members, which you cannot yet access.

Best would be to 'upgrade' your registration to full membership now you own a 260.
Send a PM to Tim asking for a questionnaire and giving him your email address for reply. Membership is free to those already members of the MGOC, otherwise you would have to apply to join at least. If for no other reason, opening up the "treasure chest" of 260 information here is worth the £50 for MGOC membership. Club shop is then open to you too.

David