Dayum Jones, you're almost as quick as I am.
Well it would be helpful to know if it has done this since he owned it, or if it started after a modification or maintenance event. I don't know Mustangs particularly well but I'd start with plugs/wires/distributor cap. So this is random, only at higher throttle openings, and there aren't any other symptoms? No hard starting, poor fuel economy, bad idle, poor throttle response, etc?