NEED HELP...Car shuts off when put into drive..

[Radlark]

Okay folks just like the title says 68 Impala SBC 350 trans, no recent work done, but the other day I let the car warm up (no LS power ..YET) and it idles fine in park but I goto to put it into drive and give it gas and the car shuts off????

I checked for vacuum leaks and nada, I can rev the motor and it revs up smoothly......I know this is kind of vague but literally this just started happening no warning at all previously ran great. Trans vacuum modulator also checked.