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As you migrated your data from your old iMac, maybe it's a software problem that has been transferred to the new one.
In this case, I recommend you to erase the disk and reinstall OS X:
1. Press Command and R keys while your Mac is starting to start into OS X Recovery.
2. Open Disk Utility, select Macintosh HD in the sidebar, go to Erase tab and erase the disk.
3. Close Disk Utility and install OS X. Don't restore the backup
yes - possible...it initially seemed absolutely fine when I migrated everything but then started once time maching had completed it's back up.
what happens to all my additional software if I erase the disc as you advise? will it be lost? (I have office for mac and logic pro which came across from an older mac book pro and I no longer have the installations discs)
You will lose everything. If you want, you can try to set up your iMac as a new after erasing the disk (without restoring the backup), and after you have installed OS X, open Migration Assistant and restore your data from the backup, but only select the data you need (apps, documents, downloads...).
Migration Assistant will create a new user with the data, so log in this user and check if the problem appears. If not, open System Preferences > Users & Groups, select the other user and press - button to remove it