If you’ve already looked for new drivers and software at the Web site of the camera manufacturer, than try resetting your Firewire port. Sometimes Firewire ports get “hung up” and need to be completely powered down and reset in order for devices to be detected again.
First, shut down your Mac, unplug all FireWire devices, and then unplug the power cord and leave it unplugged for 3 – 5 minutes. Then plug it back in, start the Mac up and see if your camera is detected via the Firewire port. (This is from Apple Knowledge Base Article HT1317.)
If that doesn’t work, there’s something else you can try: resetting the PRAM, according to this Apple Knowledge Base Article 1379.
The PRAM holds system settings when your iMac is turned off, and sometimes the settings get corrupted. To reset the PRAM, you need to press and hold the Command-Option-P-R keys as you start the iMac and hold them down before the gray screen appears. So find all the keys first, and then press the power key (on the back of the iMac) and then get those keys pressed down. Hold them down until you hear the startup chime for the third time, then let up on the keys.
Resetting the PRAM will reset your clock and other settings, such as keyboard repeating, etc., so you will have to reset them afterwards.
Also, if none of those work, look in online forums for other people who use the same camera and see if they have problems. There might be some work arounds to get your camera to work consistently.