Hello, my son says his 2004 mustang is

Sounds like a misfire , plug or coil breaking down . Does the check engine light come on or flash ?

Sorry old son but your clutch sounds like it has gone to the big junk yard in the sky reserved for dead clutches . When a clutch starts to slip it will continue to do so until that is all it will do . solution a new clutch is required .

When the "spits and sputters" happen are you in low rpm? Can you "by-pass" the sputtering by accelerating? Has your gas mileage suffered?

How many miles? Because if you haven't, consider having your fuel system inspected. It may be time for a fuel filter replacement.

Try changing the inline fuel filter. It may be plugging up with gunk from the tank when you accelerate after running. If the gunk is settling in the lower part of the inline filter, this allows some fuel to flow, but when you accelerate or drive for an extended period, the gunk gets shoved against the output end blocking fuel flow. It also could be picking up some water in your tank, if the tank is allowed to be low on fuel.

check fuel pressure if fuel pressure is low u have either clogged fuel filter or bad fuel pump

Fuel filters normally cause this kind of problem. Check the fuel filters. If it sputters when you accelerate hard this is the most common sign. If the filters are fine run a fuel system cleaner in the gas and then change your spark plugs.

check whith scanner for codes,it coul be missfire,it need to be diagnostic

if you're trying to hold the idle over 3500 , you're going to hit the Rev Limiter, Einstein. Racing cars will lead you to no good , son. If I'm wrong or missunderstanding you please clarify.

Have 2004 Nissan Quest. After 40 mph Van runs rough and there is a loud rattleing noise that sounds like an old 4 bbl carb waaaah, but ther is no code