The hard starting problem could be due to a couple of things:
low fuel pressure and/or an engine vacuum leak.
Your engine should have 32 to 47 psi of fuel pressure with the engine idling. You need to check it with a gauge. If the pressure is low, the problem could be a leaky fuel pressure regulator, or possibly a weak fuel pump (you said you replaced the filter already).
The regulator can be checked by watching the fuel pressure gauge while you disconnect the vacuum hose to the regulator. Fuel pressure should usually go up at least 5 psi. If you see any fuel inside the vacuum lose, the regulator is leaking internally and must be replaced.
The fuel pressure regulator is located on the fuel injector rail (the round thing ont he endi with a vacuum hose attached to it)
I would also carefully inspect all of the engine's vacuum hoses, intake manifold and throttle body gaskets, PCV and EGR valves for possible vacuum leaks.
Finally, a bad airflow sensor could also be causing your hard starting problem.