Miss firing with ck light

I have a 97 mustang with a 91 Sc engine in it. It is a auto car with 373 gears and 7% over pulley and a 70 mm tb. It has started missing when accelerating under load then it clears out. The check engine light cam on and the code is lean on both banks. So do I start with the checking fuel presser or with the electric system or the vac system. It shows 16 inch of vac at 750 rpm in neutral and around 14 inches in gear. The boost max's out at 12 pounds. The car has always seamed a little ruff when in gear at idle, but ran ok.
Where do I start looking, by the way the plug wires are 21/2 years old with around 6000 miles on them and the same with the plugs. The wire are motor craft and the plugs are auto light. Also some times I have to hold it to the floor to get it to start, I think it has done that may three or four times.
Thanks for your help.
Dave377

dave377 said:

The check engine light cam on and the code is lean on both banks.

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I'd say the vacuum leak gremlin paid your house a visit...

Have you gone thru any process to check and eliminate yet?

I assume you are running the Mustang ECM and OBD2? If so, hook up a scan tool and look at the data stream to see what your fuel trims are at idle, and if they get better or worse as you rev the car up. If they get better, then it is probably a vacuum leak somewhere. If they get worse, it could be low fuel pressure, or a dirty/bad MAF or something like that.

Thanks for the info guys I will try to take a look at it this weekend.

Ok guys I checked the fuel press and at idle it is 32 psi with the vac.hose on and 40 with it off' It holds press good after shutting off engine. I also tried different vac gage and it still reads 16 in vac at 800 rpm in neutral and 12/13 in gear at 750. I using starting fluid around the blower and air tubes and the intercooler and the vac hose including the pcv valve and the booster there was no changes. I am going to get some cleaner for the mas air meter and change the fuel filter. I also looked at all the plug wires and they look good and they are on good and tight. If the mas air cleaner don't help I am going to take the plugs out and check them and run a compression check to make sue I don't have a bad valve or valve spring.
After I get this problem cured I am going to install a ported late model blower case and mp intake plenum, along with 42 pound injectors and a 76 mm mas air that is calibrated for a supercharger draw through using 42lb injectors. I also have a npd fmic to put on it. so looks like I have plenty to do after I get it running rite.