99 ford mustang 3.8l

I have a 99 ford mustang 3.8l I bought not long
ago, then man i bought it from said it had been
sitting up for 5 months, I have changed the plugs,
wires, fuel filter, air filter, oil, crankshaft sensor,
camshaft sensor, dropped the gas tank and
drained old gas out, brand new battery, checked
and replaced all fuses, the car will crank but will
not start unless I pump the gas, if I hold the gas
down it back fires and won't start but I can pump it
and will start, once its running it idles good but
when I put it in reverse it trys to die then I put it in N
and the RPMs shoot up to 2500, put it in drive and
it hesitates a minute then shifts in a will spin the
tires without me pressing the gas but then trys to
die unless im giving it gas any ideas on what could
be causing my problem? Also check engine light
isn't on so it isn't throwing any codes, any advice is
much appreciated