Apple Inc.'s online support forum has been filling up with complaints from iPhone 3G and iPhone3GS users that upgrading to new iOS 4 operating software has led to irritating glitches including sluggish performance, drained battery life and overheating.

But, while Apple said it was investigating complaints from iPhone 3G users, it said it had not received any actual reports of issues directly from iPhone 3GS users.

“If customers are having problems, we advise them to contact Apple Care for assistance,” said Natalie Harrison, an Apple spokeswoman.

Chris Squires of Bakersfield said he called Apple immediately when his entire contact list was wiped out after upgrading his iPhone 3GS to the iOS 4. The Apple employee couldn’t figure out what happened, he said.

“It doesn’t sync up to my Bluetooth anymore,” Squires said. “If I’m in the middle of a text or e-mail or website, it’ll freeze and then I’ll have to power it down. I never really had to do that with the regular 3GS.”

The new operating system, which was rolled out with the iPhone 4 in June, boasts more than 100 new features. Apple has said that the software is compatible with both older generation iPhones: the 3GS, which came out last year, and the 2-year-old 3G.

Tim Bajarin, an analyst with technology research firm Creative Strategies Inc., said it was common for new software to have compatibility issues with older devices.

The majority of people having problems are iPhone 3G owners, he said, because the iOS 4 was built around the minimum requirements of iPhone 3GS technology.

“There are going to be instances where some programs that were written for a higher level functionality might not work properly on the 3G and that could cause everything from battery drain to system crashes,” Bajarin said. “But, to be fair, those phones are out of contract and under normal circumstances people upgrade.”

Therefore, it’s an industry wide procedure to build new operating systems based on two-year intervals, he said, which is why some iOS 4 features aren’t available on the iPhone 3G — despite it being fundamentally similar to the iPhone 3GS.

kristena.hansen@latimes.com