I have the Miller 375 Extreme and it has been a great tool. Couple of things does your model have the 110 and the 220 plug?
Mine does I think there were some older 375's that only had one or the other.
I set my shop air to 100 psi at the compressor and let the plasma cutter do the rest never had a problem.
Good Cut Quality requires a few things, clean dry air - correct amps for the material being cut, good ground - and correct cut speed and standoff distance.
Make sure your consumables are new to start to eliminate them as a cause of problems, There are nozzles that are set up to be dragged on the material and others that need a standoff distance about 1/8 make sure you are using the correct nozzle for what your doing.
If your doing everything else correctly your most likely cutting too fast. If you cut too fast the torch does not cut through everything and you have some parts that are still connected.
Watch your sparks if you are getting lots of sparks on top you are cutting too fast. it takes a little practice to get the speed right by hand.
Its a great machine and that what a steal of a price
CNC Plasma Project: www.cncplasmabuild.com
CNC Router Project: www.cncrouterbuild.com