I have gone through the previous messages about this issue:

3 short + 3 longer + 3 short beeps means EFI ROM checksum failed. I downloaded and created Firmware restoration CD 1.6 for iMac7,1 from Apple support.

Turned power on and iMac sucked in the CD and ejected after 1 minute and started beeping cycles again.

Questions:

1. I burned the CD on Windows with PowerISO. Should I try burn the CD on Mac instead?
2. It seem doing something with the CD. Or it's hardware issues with the ROM or board that firmware refresh won't fix?

Any feedback welcome. Thanks.

ps. I remove the screen and couldn’t see any bad connection and also replaced the lithium battery too.