I’ve got HWinfo and Memtest86
hwinfo is the most useful, 'cos it shows you almost everything, you can run it alongside your games and you can see the min/avg/max values. You can check the power limits on the CPU and the graphics card and look at the other sensors to make sure it is behaving as expected.
The 3D Mark test I mentioned awhile ago is also helpful, since it shows you the clocks over time in a graph.
I believe GPU-Z is able to show you the current bus speed, which can help to diagnose PCI-E issues, like if your board is stuck at 1 or 4 lane instead of 16 lane. Though, cards are capable of downclocking, so you need some load to check that.
Event viewer has a lot of useless stuff, but also occasionally shows something helpful if you can check the timestamps when stuttering occurs.
Task manager is a simple tool, but it can at least show you if some of your parts are sticking at 100% load for no obvious reason. E.g. with SSD problems.
Note that RGB, motherboard and monitoring software (even mouse software) can actually
cause stuttering, so I'd be wary of using too many of these things at once.
I assume you've already tried setting the PCI-E gen down to 3 or 4 in the BIOS, but if you haven't, I'd try that because it was a common issue when these cards launched.
You had afterburner or something like that running when you see the lows drop, right? I assume there wasn't anything obvious, like high memory or CPU usage at those times?
I'd say that testing the memory for faults is unlikely to be helpful here, because faulty memory usually gives you BSODs rather than just stutters. SSDs with problems can cause freezes, stutters and hangs, so worth checking the smart data with something like CrystalDiskInfo. Bad SSDs causing stutters is likely to show up in the event viewer too.
You tried with EXPO/XMP off already right?
Did you try a different monitor at any point?