Robert;

Refer to the "H" diagram in the article: Electrical ramblings: http://www.intelab.com/swem/elecramb.htm
and presuming your ampmeter is hooked up correctly as shown in the diagram, you can see that the cause of the heavy charge or discharge current indicated is quite likely to be the battery itself, since the (starting)current path 1. and (load)current path 2. dont even go through the meter! Car batteries have a few different failure modes (which typically involve their inability to store or give back stored power), but the most deadly to your alternator is an internal intermittent short which would show up as a high charge current. As far as the high discharge current...try to figure out where the current is going (whats getting hot, try pulling fuses etc.)

My advice is: The next time your car exhibits this symtom, have the battery checked for an internal short immediately!

Merry Christmas Brickboarders!