I suspect you're hitting a SATA interface issue.
Your iMac has a SATA I (1.5 Gb/s) I/O interface, as such you really need to install a drive that is SATA I or can be adjusted to run at SATA I speed.
Your Intel 240 SSD 530 series SSD is SATA III (6.0 Gb/s) only.
Sadly, this drive won't work internally in your system. It will be hard finding a new SSD that runs at SATA I speed (we've given up) all you'll find is used ones.
Instead we moved on to SSHD hybrid drives. While I understand your desire to have a wicked fast SSD drive a SSHD will get you dam close to the same performance without the lag issues traditional HD's have. So you get the zip of a SSD and the deep storage a HD offers in one device. Here's the Seagate drive info: desktop SSHD and here's the spec sheet: desktop SSHD specsheet. Note the line: SATA Transfer Rates Supported (Gb/s) 6.0/3.0/1.5. Make sure what ever drive you get it has the same ability as this one.
One last issue here is you do want to make sure your firmware is upto date. Follow this Apple T/N: EFI and SMC firmware updates for Intel-based Macs.