sounds almost like your cards are in ULPS because the 300mhz is the idling clock speed(crossfire bridge connected? crossfire enabled from catalyst?). Have you tried manually setting the clocks with trixx? Have you re-installed the drivers? My windows never survives a card swap or slot change etc. Drivers needed to be re-installed every time I changed something.
I didnt re-install the drivers after my hardware change, they are identifying my cards right. My 5850 is, shown in CGminer, at 400 Mhz clock rate.