I find this hard to believe since hp has been using the PMC sierra for a long time and the p420 is solid. I'd give it a shot - it could be the operating system or other environmental issues. REMEMBER these cards need next-gen cooling, ideally active dual fan or heat-pump. the stock cooling with G6/g7 or 11th gen (r610 gen) are not sufficient in many cases. The entire card uses 14 watts. the cpu maybe 10-12 watts, but it is not easy to cool it! They need pure copper or heat pump technology.
I guarantee you, if you run the cards hot, they will clock down and destabilize.
The 9260 could run pretty much open-air in a desktop, the 9266/9271/P420/71605 - no dice!!