Well... I've come to the determination that this must be some kind of hardware compatibility or fault issue. I re-flashed the BIOS that made no change, so finally in desperation I did a full format and install of windows 10, and am still facing the same issue.
Just to be sure I wasn't crazy I dug out an old laptop running Win 7 and ran pingplotter on it simultaneously with the problem desktop.
Here is the results form the old Win 7 laptop;
Target Name: att.com
IP: 144.160.155.43
Date/Time: 4/14/2018 3:12:43 AM - 4/14/2018 3:22:43 AM
Hop Sent PL% Min Max Avg Host Name / [IP]
1 244 0 1.00 1382.00 9.18 attwirelessinternet.wirelessi
nternet [192.168.1.1]
2 244 0 1.00 1337.00 8.85 10.13.215.122 [10.13.215.122]
3 244 0 70.00 1366.00 86.11 172.26.96.161 [172.26.96.161]
4 244 0 72.00 1316.00 86.07 172.16.126.124 [172.16.126.124]
5 244 0 72.00 1282.00 88.45 12.249.2.49 [12.249.2.49]
6 244 0 75.00 1242.00 90.10 12.83.180.82 [12.83.180.82]
7 244 0 73.00 1200.00 94.24 12.122.110.5 [12.122.110.5]
8 244 0 76.00 1142.00 87.52 12.91.205.18 [12.91.205.18]
9 244 0 77.00 1519.00 119.13 att.com [144.160.155.43]
No packet loss, average ~120 ping time. Nice!
And here are the results from my desktop with a fresh install of Win 10
Target Name: att.com
IP: 144.160.36.42
Date/Time: 12/5/2020 1:20:08 PM - 12/5/2020 1:30:08 PM
Hop Sent PL% Min Max Avg Host Name / [IP]
1 244 87 1.14 2.18 1.70 attwirelessinternet.wirelessi
nternet [192.168.1.1]
2 244 87 1.97 4.53 2.86 10.13.215.122 [10.13.215.122]
3 244 87 71.82 128.56 86.11 172.26.96.161 [172.26.96.161]
4 244 86 72.58 112.16 81.20 172.16.126.124 [172.16.126.124]
5 244 85 75.31 135.54 86.50 12.249.2.49 [12.249.2.49]
6 244 85 120.48 158.56 131.24 12.83.180.82 [12.83.180.82]
7 244 86 121.15 156.61 131.42 cgcil21crs.ip.att.net [12.122.1.173]
8 244 86 120.26 239.00 138.90 12.123.159.17 [12.123.159.17]
9 244 86 127.61 152.23 133.73 12.249.81.194 [12.249.81.194]
10 244 89 132.69 151.72 139.47 att.com [144.160.36.42]
Basically 90% packet loss.
Also ignore the time stamp on the laptop one, I didn't set the time on it, these where run at the exact same time, both on the same wifi. Also it makes 0 difference if I plug into ethernet instead of wifi.
I suppose I'm going to start looking for a christmas deal on a new MB, CPU and RAM. MSI says this MB is fine with Win 10, but something in my system is obviously not happy with it.