Can an ECM act funny and not work right at all times before going completely out?

If it won't throw a code for it, can it still be malfunctioning?

Is there a way to test an ECM without installing a new one first?

I have a thread in the help section on a problem I'm having. The thread is titled: Mid-Range Fuel Miss, 2002 CTD

 

i (along with HOHN) had to replace our ECM's. i got one of the dodge techs to print me off the pins and which goes to what. Then i determined that it was my ecm that was bad