prehistoric motor control trouble

Once upon a time, long long ago, there was no such thing as VFD's. Its scary I know, and now Ive stumbled onto a Reliance 3hp DC motor control. When I first started putting the machine back together it worked great, the motor ran smooth and I was getting excellent response from it. Then one day I fired it up and the system is making a popping noise. The noise is more pronounced at lower speeds and almost goes away at higher speeds. If you put any load at all on the motor it trips the breaker. The noise coming from the control almost sounds like a switch clicking on and off around 2 times a second at about 500 motor rpm. and about once every 3 seconds and much more quietly at full speed.

As for what I know:

I've isolated the problem to the control because the motor runs fine when I hook it up to an old control I had laying around. I have 200 clean volts at the field. The armature voltage also looks good and goes up and down with the pot like its supposed to. Input voltage to the control is also ok. I also have the manual and nameplate for the drive if I've omitted any vital information.

I think I have a bad contactor or transistor but I'm not real sure where to go form here. Has anyone ever experienced any of these troubles? Or think they may have an idea whats going on?

Thanks a ton!