2019 27 in 5k iMac Shuts down while using Lightroom (HELP)

[#1]

2019  27in 5k IMac

Version 10.15.7

3.6Ghz 8-core Intel Core I9

64GB 2667 MHZ DDR4

Radeon Pro Vega 48 8GBOk 

I have been having issue with 27 iMac shutting down while using Lightroom.
This happens during normal use. Applying presets adjusting white balance, brightness.
Nothing major . Times very, but it may be 20 min or 30 min before it shuts down. Sometimes less then a min.

I have been to apple store. On first visit it showed I had bad memory and it was replaced. Sadly issue still there. On second visit they ran complete diagnostic and everything came back normal no issues detected. At this stage I factory reset my Mac . I have purchased 64gb memory from crucial. I had the original Apple memory replaced. Unfortunately the issue is still there. Something is wrong but I cannot figure out what. I am turning to all of you for some guidance.

[#2]

Computer crashing is always a bad driver, or a hardware malfunction. Re-install all drivers. Run diagnostics on ALL hardware, and make sure the cooling system is working properly (all fans running, no vents blocked, case vaccuumed out)