965 ascent over a run incorrect....

Strange , mine is accurate. Is the base altitude correct i.e. the starting altitude, Maybe try this?

www8.garmin.com/.../GUID-BC734846-01A7-4F33-86D4-DFBDBC06CDB4.html

Does the elevation graph on Connect from the activity show strange spikes? Maybe give us a screenshot?

Baseline looks correct, but there are spikes in both directions. (Never had this with my 235).

Not sure why/if it may impact this , but is 3D Pace and 3D Distance setting toggled OFF?

Yes they are both off

Sounds like messed up hardware. One thing you can try is to clean the entry hole for the barometer, it is the small hole on the back side centrally at the bottom. Maybe something got stuck in there? Don't use anything aggressive to clean it, warm water and a hand brush or maybe a thin paint brush, maybe some mild soap. Otherwise, get in contact with Garmin and have it exchanged, it sounds broken to me.

Thanks. I read that somewhere. Just don't think that I should have to clean a brand new watch. I've asked Garmin for their response and the supplier. But I agree, sounds broken to me too...

Yes, you can surely just exchange. But as exchanging is a lot of hassle and you are without a watch for a while, I personally would at least visually inspect that barometer hole to see if there is something jammed in it first :D