Specs look generally fine. See if you can confirm what make and model the PSU is; a good quality lower wattage PSU is preferable to a low quality high wattage PSU.
Back to the ingame monitoring. If you haven't, download MSI Afterburner and log/monitor the following: CPU (all cores/threads), GPU and RAM frequency and usage; CPU and GPU temperature; fps and frame times; and page file. What we're looking for are points in the graphs which correspond to the lag you mention (either dip in fps or longer frame times).
Initial thought was you upgraded to a higher resolution and higher refresh rate monitor, which is a possible explanation but not conclusive.