Mouse Trackpad pointer freezing on startup when second monitor connected


I’m running a late 2015 iMac 27" 5K. At launch, the the system boots to the login screen, the pointer freezes and lags severely - or once in a while, the computer reports the trackpad can’t be found. The trackpad is the current model that shipped with the computer.

I’ve narrowed it down to having my secondary monitor plugged in via the thunderbolt/display port connector because I found mention of this problem via Google - people with secondary monitors plugged in having mouse freezing problems. The machine starts clean if the monitor is not plugged in with no mouse lag. If I ignore the problem and use my arrow keys and return key to log in, the problem goes away within a moment or two of the desktop launching. After that there is not a single problem, ever. Unit I reboot.

The system is running the latest High Sierra but this problem dates back about a year to when I had Sierra, although on Sierra it was more random. Apple were fantastic in trying to help me. Even a full, clean reinstall made no difference. As well, I’ve tried replacing the trackpad and been through all sorts of diagnostic tests and reports back to Apple. At that time, they couldn’t ID the problem and it seemed to happen without the monitor being connected. Now, with High Sierra, it’s related only to the second monitor plugged in.

If I run a corded mouse, there’s no problems. If I unplug the secondary monitor - a Dell u2711 - then there’s no problems. But that’s not how things should be.

Suggestions at this point would be much appreciated.