As usual, don't forget to check hall effect (in distributor) or bell housing rpm sensor; depending upon which year you have (see FAQ). If you have the coin, replace the fuel pump relay, radio suppression relay (unless its an older model without one) and rpm (or distributor if hall effect) if they have not been replaced. This will save you the same headache down the road even if this is not your problem now. Whenever my 740 or 940 has done what you describe, it has been the relay or hall effect (or rpm sensor).