1650 super issues on 3rd gen i5 optiplex system

I got a 1650 super for my dell optiplex 9010 with an i5 , 8 gigs of ram, 450 watt power supply and windows 10. After installing the card, the system is working and I'm getting decent fps in games. But something I noticed was that the computer became a lot slower and now whilst using it, it randomly starts lagging, hangs and then the screen turns blank. I have to force shut it down and restart it after a while. It sometimes happens when I launch a game and sometimes happens completely randomly. I have reinstalled all drivers using DDU, reinstalled windows, updated motherboard bios and any other solution I found online. The system works fine without the Gpu when used with the motherboard display ports. I had the GPU checked on another pc and it seems to be working fine on that. Could this just be a compatibility issues as my optiplex system is pretty old and low spec and it's motherboard uses legacy bios. The computer shop where i had the system checked are saying that the 1650 super is too much for the old 3rd gen i5 system and that I should just get a higher spec motherboard and cpu. I am really confused as to what to do cuz I have seen people online running 1650 super or even higher spec cards on old optiplex systems. Any solution or is upgrading my cpu and motherboard the only choice.