I usually shoot in AV for landscapes, It has always worked great, since I am just starting with portraits, AV does not work well, so whats your problem?
I'll take a stab at a "problem"

#1 if you're using your strobe lights you should be shooting in manual. If you're shooting in any modes other then that, especially idiot-proof setups your exposure will probably be off, AND HERE IS WHY:
Imagine you're in Av, you set you ISO 200, f/8 and camera adjusts the shutter speed - lets say 1/60 (if you're fortunate enough in-doors). BUT all that what camera see and doesn't know about your external lights that you'll be using - so you'll probably burn out your subjects. Then as you play with zoom, again camera will readjust the shutter speed to let's say 1/15 and again you'll have beautiful WHITE dot rather then subjects. So that's why you should be shooting with strobes in manual UNLESS you have means to use either camera or some other remote and adjust power outages on external units (aka Nikon CLS)
#2 on your 50D when you plug in the PC cord and plug that into ONE strobe, you have to position other units so built-in eye sees the flash (either direct or indirect/bounced) and fires accordingly - the light with the cord will be so called master.
#3 - going back to original point -
what the hell are you doing shooting in portrait mode!? Its same as idiot-green and once again - camera will NOT know that you're shooting with strobes thus will popup your built-in flash.
With all that in mind, step one is (and I can't emphasize it enough)
LEARN YOUR EQUIPMENT only after will you be more efficient and proficient in your skill.
Good Luck