My MBP will sometimes drop 4 of the 8G of RAM I have after recovering from sleep or a reboot. One (or more) restarts later I'll have all 8G again. Is this a memory stick going bad? Could it be something worse?
Yes it could easily easily be a bad connection on the ram chip or just bad connection of the RAM stick with connector on the motherboard. Try to clean RAM connectors with a gum eraser first.
To test memory under load you may also use Memtest86+ CD if your MBP is Core 2 Duo one or Apple Hardware Test from included CD if yours is "Core i" one.