Amazon.com Inc.'s Web site suffered sporadic outages Monday, just days after unspecified issues knocked the online retailer offline for more than two hours.

Keynote Systems Inc., a California-based company that monitors Web site performance, said the latest troubles started shortly after 1 p.m. EDT and lasted at least an hour.

Keynote's automated probes were able to reach the main Amazon.com site as little as 30 percent of the time. Even when the monitors did reach the site, they faced delays, said Shawn White, Keynote's director of external operations.

The probes also found problems with the British Amazon.co.uk site, but other country-specific sites appeared to be functioning, White said.

Amazon spokeswoman Patty Smith described Monday's troubles as "intermittent" and said they did not affect Amazon Web Services, a separate offering that provides pay-as-you-go data storage to other companies such as the photo-sharing service SmugMug Inc. and Web e-mail provider Mailtrust.

On Friday, Amazon's site shut down for more than two hours during the business day, giving an error code to anyone visiting it.

"Amazon's systems are very complex and on rare occasions, despite our best efforts, they may experience problems," the company said in a statement explaining Friday's outage.

Smith said Monday that the statement was not meant to rule out any specific cause, including an external, denial-of-service attack intended to overwhelm Amazon's servers, as some blogs have speculated.

But Keynote's White said the "service unavailable" error message returned and the speed with which the site went down suggested an internal systems problem rather than an outside attack.

A similar "service unavailable" message greeted some visitors Monday, though at times the generic message was replaced by an apology and a promise to restore service quickly.

Outages at Amazon are rare, though the retailer had brief disruptions in 2006 because of a Thanksgiving Day sale on Microsoft Corp.'s Xbox 360 video game machines.

Earlier this year, Amazon had trouble with Amazon Web Services. Several companies temporarily lost access to their own files when that system went down.

Smith said she could not speculate on how the latest outages affected sales.

Some customers might return to shop later; others may visit a competing site or buy offline instead.

In April, Amazon had more than 58 million U.S. visitors, according to comScore Inc. Last quarter, the company recorded $2.13 billion in sales in North America.

Shares of Amazon lost $1.20 or 1.5 percent to close Monday at $79.43.