Could be points worn out if you are still running points in the dizzy. I had a car that did that once, but backwards. Would shudder like hell in reverse, but would do OK in drive.
^^ yep - that's a good guess. Also be sure that the vac line for the advance is hooked up and working ok and good shape (not leaking). Could be plate in dizzy not advancing properly or flakey/loose/worn
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