New to these forums and can’t locate problem.
So I have a brand new alternator and battery, got them 2 days ago. When I start the car, my lights flicker for a bit and stop, but come back every now and then, the car stays above 1000rpms when in park, and stays high/fluctuates when in any other gear (not moving, just in the gear). When I turn on brights in park the rpms shoot down pretty low and it sounds like the engine is struggling. Is the alternator just not strong enough or maybe poor connections....? I really love my eclipse and hate when it has troubles

Oh..and I did scan it and it said code P1500 but I have brand new alternator and no check engine light on.
Mitsubishi Eclipse GT 2001