Were you able to check the vac line to the modulator? I'm not sure if it could effect it that dramatically, but maybe it got unplugged or torn and so the modulator thinks the throttle is wide open.

It should be seeing a decent amount of vacuum at idle and during gentle driving. Beyond that, yeah, you may need a new one. Mine experienced similar symptoms when the rear pump died, which can be replaced.

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1967 230S automatic
Boston, MA