When stress testing with Aida 64, temperature spikes is very normal especially since Aida64 cycles between different tests whilst stress testing.
As for indications if your overclock is unstable; you're trying to stress test for a few hours without getting a blue screen or system reboots. Another thing to look out for is thermal throttling which occurs if your voltage setting is too high for your CPU cooling solution to handle.
CPU utilization fluctuation can mean a lot of things. It's either that Aida64 is acting up which is very unlikely. This could indicate that your CPU is thermal throttling due to package temperatures or VRM temperatures are too high.