The top-of-the-line 500MHz Power Mac G4 will ship after Apple’s November target because of "performance issues", industry insiders claim.

Motorola's Semiconductor Product Sector is working to resolve "errata" that affect its new G4 processor when run at speeds of 500MHz or higher. According to Motorola's own schedule, a fix isn't due until December.

Motorola released Revision 2.2 of its G4 processor this summer, followed closely by Revision 2.6, which is shipping in the initial, 400MHz version of the Power Mac G4. Sources said - and Motorola's Technical Support Hotline confirmed - that both revisions 2.2 and 2.6 contain errata that can be avoided only by keeping the processor speed below 500MHz.

Sources said that the problem - which only arises when the G4 is run at speeds of 500MHz or higher - can result in corruption in the processor's data cache. Motorola's recommended workaround is to enable the "GlobalWaitR" register in the processor - which, while preventing the problem, slows timing throughout the chip.

Motorola's Technical Support Hotline confirmed both the existence of the errata and the workaround, which it acknowledged entails a "speed hit".

Although the G4 processors in shipping Power Mac G4s contain the errata, their sub-500MHz speeds keep them from encountering the corruption problem, sources said.

Indeed, another source said, this issue might never evince itself in Macs, since the OS doesn't manipulate data rapidly enough to cause the problem - the glitch would more likely effect more-efficient embedded operating systems. Even if data corruption should occur, a source said, the result would be nothing more than a system freeze, easily fixed with a restart.

"That kind of errata isn't unusual for new ships from any manufacturer," said Keith Diefendorff, editor in chief Microprocessor Report. He said that Motorola's warnings don't necessarily portend serious problems: "Motorola, as a company, is relatively conservative, and they like to have everything perfect."

Sources said Apple is telling a different story to its resellers and customers. In a report to dealers last week, Apple reportedly noted "intermittent shortages" of the 400MHz and 450MHz Power Mac G4 systems and listed an "expected" October ship date for the 500MHz configuration. Sales staff at the Apple Store, by contrast, said the top-speed model will be available by the end of November.

Apple declined to comment.