Owners of Apple's new iMacs, including the pricier 27-in. model, have reported major performance problems with their machines, including extremely sluggish playback of Flash-based video, according to posts on Apple's support forum.

"I go to YouTube and all the videos play really jerky, with sound popping ... so I opened Activity Monitor and Flash Player (only thing not running 64-bit) is spiking to 105% CPU usage every few seconds," said a user identified as "Subcide" last week in a message that kicked off one of two threads dedicated to the problem. "It's straight out of the box, haven't even installed a single app on it yet."

Subcide posted a video to YouTube that showed the jerky Flash video and the iMac's processor hitting 105%.

Others jumped into the thread with similar reports. "I have the same issues with Flash video as already mentioned ... but also add that I see performance issues outside of the browser," said "blurden" in a message last Monday. "For instance, something as simple as resizing a window in Adobe Photoshop CS4, and even in Finder."

"My 27-in. iMac is having major performance issues," complained "Kermit262" on the same thread. "It's not just Flash. iPhoto slideshow transitions are choppy, Pandora streaming music skips and cracks up, game performance is slow, iMovie videos are choppy -- it's a mess. I've worked with AppleCare for hours -- cleaned up libraries, zapped the PRAM, reinstalled the OS; but it's still not fixed."

Others reported that the performance issue also affects some of the less-expensive 21.5-in. iMacs that Apple introduced Oct. 20 .

Most of the messages in the second thread were duplicates of ones posted in the first thread.

Users struggled for answers or explanations, and posed possible solutions that ranged from reinstalling Snow Leopard and installing Microsoft's Windows 7 in a Boot Camp partition, to rebooting their iMacs and disabling the machine's AirPort wireless feature. But the lack of a clear answer, even after trying those tricks and others, led "HomerT" to conclude that a fix from Apple was required.

"The more I use this computer, I'm starting to think this isn't a Flash issue or a Snow Leopard issue but may indeed be a computer hardware problem," said HomerT. "I just don't know if I have a defective unit or if all these new iMacs are in need of a firmware update. I'm so frustrated right now."

"I definitely think that this is a hardware bug of some kind," added "tgriff7" in a message posted yesterday. "There seems to be a leak of system resources that goes on the longer I work on my computer. Flash seems to be trigger it, but I certainly don't think, from what I've seen using this machine pretty heavily over the past week, that this is a problem limited to Flash."

Others have already thrown in the towel. "I think I've run out of patience," said Kermit262 in a message Wednesday. "I have a perfectly working iMac 24-in. sitting next to this one, and after getting off the phone with AppleCare, I looked at it and thought, why am I going through all of this? I plan to take the 27-in. back today, and buy at a later date when all the bugs are worked out."

A few users held out hope that an impending update to Snow Leopard, which has been seeded to developers and testers, according to the popular AppleInsider site , will solve the performance problems. Earlier this week, Apple asked testers to provide feedback on, among other things, the graphic drivers included with Mac OS X 10.6.2, said AppleInsider.

Not everyone has seen their iMac crawl. "I found this thread Tuesday and put [my iMac] to sleep one time for a minute," reported "Moses195" on the original thread. "Problem went away. Since then I've done all the stuff I had been doing but no problems."

Apple did not respond to a request for comment on the issue, or to questions about whether a software fix was possible, and if so, whether one is in the works.