Those CPU spikes…do they happen regardless of aircraft, at any airport, etc.?
One time I had stutters that would happen at regular intervals (not this often). I found out it was dashost.exe talking with my smart tv over the network. I had to set up a firewall rule to prevent traffic from that IP address and dashost.exe. Not saying that’s necessarily the case here, but it gives you an idea of how wide a scope you should be looking.
Use broad strokes at first: disable all addons in community folder, disable external software running with MSFS. Close background programs one by one while the sim is running to see if one of them is the culprit. Process Lasso gives you the ability to sort programs/processes by CPU avg. utilization. If it is an external program maxing out CPU utilization that frequently, it will likely be near the top. Some people use Process Lasso to help with performance; I use it because it is better than task manager for solving this type of problem.
Use Resource Monitor to find possible network utilization corresponding with the spikes. Also look in your windows update history for recent driver updates that may have corresponded with when the problem began.