Unresolved iMac performance issue after multiple attempts with Apple support

I have taken the iMac to my nearest store three times on advice ( thats 200 mile round trip and endless days without the computer), I have allowd Apple to reset remotely my computer 7 or 8 times and, most recently been told Apple know what the issue is ( issue with a processor they were using) but won't sort it for me. The chap could happily tell me he bought the same iMac around the same time and had to sort it himself at a cost of £92.. My iMac is so slow it's not worth turning on. All the signs are good ( less than 10% space used) but the endless guarantees that they's not let me down that this time they would keep in touch until it was sorted.......broken promise every time. What more can I do?

[Re-Titled by Moderator]

Original Title: I have spent 4 years nearly, written over 172 emails, spoken to 14 employees, had promises from 7 of them that what I was experienceing was unacceptable and they'd sort it , guaranteed, ....how do I escalate this to someone who can resolve my issue?