Several online retailers say they have experienced two or three e-commerce site outages on the Volusion Inc. e-commerce platform in recent weeks, increasing their concerns about the ability to handle peak holiday traffic.

“There have been three outages during the holiday rush,” one retailer on the Volusion platform said Friday. The retailer, who asked to remain anonymous, said his site was down for about 25 minutes on Dec. 7 and twice on Dec. 13. Those outages followed earlier ones in November on Volusion sites.

Other retailers, who also asked to remain anonymous, said their Volusion sites were down at least twice within the past two weeks and noted that such experiences were widely noted in online forums. “From the various Volusion merchant groups and message boards I frequent, it seems like a pretty large number of their stores were affected,” one retailer said. That retailer also said that Volusion has repeatedly blamed down times on distributed denial of service, or DDoS, attacks. Those occur when perpetrators attempt to knock a site offline by sending an overwhelming volume of traffic to it. Because Volusion’s e-commerce site clients operate on a shared technology platform, at attack on one site typically affects multiple sites before the problem can be rectified.

Via its Facebook page and its status update page at Status.Volusion.com, Volusion confirmed outages on Dec. 7 and Dec. 13, as well as earlier ones in November. In each case Volusion noted that outages had been rectified later the same day.

But Volusion declined to comment last week on the role of DDoS attacks in the recent site outages, or about steps the company is taking to prevent them. DDoS attacks are particularly troublesome, Volusion clients say, because Volusion does not include DDoS outages in its guarantee of 99.90% web site uptime. “The 99.9% uptime measurement does not include downtime caused by factors outside of Volusion’s control, including, but not limited to, the failure of customer or third-party equipment, denial of service attacks or force majeure events,” Volusion says in its 99.9% guarantee policy. Force majeure pertains to occurrences like tornadoes or other devastating and unusual events that a technology or service provider claims it cannot guard against.

Volusion, which says it serves about 40,000 e-commerce sites, maintained 100% uptime on all sites during the time between Black Friday, Nov.  23, and Cyber Monday, Nov. 26, a spokeswoman says. “From Cyber Monday up until now,” the spokeswoman said on Friday, “we are operating at 99.95% uptime, which exceeds our service level agreement [of 99.90%].”

Another Volusion retailer, The Office Stylist, however, says outages with Volusion have been frequent enough to consider another e-commerce platform. “At this point I don’t take note when we are up or down because it’s happened so many times,” says Sayeh Pezeshki, founder and CEO of the The Office Stylist, which sells online at the Volusion-supported site SortingWithStyle.com.