96 Blazer/Battery uncharges while driving

The battery seems to uncharge while driving. I've put a new alternator on it and a new battery. I'll charge the battery back up, start the car up, and take off driving. The weird thing is I can see it discharging on the guage mainly when I am stopped at a stop sign. If I have it in park and running it seems to be alright. Me and my dad have tried everything....someone please help.

Also when I try to take off after I have stopped and it has discharged a bit, it seems like it doesn't want to go.

When the key is turned to the RUN position, (engine not running), does the red battery light on the dash illuminate?

With the battery fully charged, all accessories turned off, engine at 2000 RPM, voltage at the battery should be 14.2 to 14.5 volts. Don't rely on the dash voltmeter, use a hand held voltmeter. Post your results.