You’ve got the right laundry list of bullet points to check for troubleshooting your stability issues.
Ruling out thermal issues is as simple as opening up the chassis and verifying if the cooling fan is operational and intact (no broken blower wheel fins or bearing noise that could indicate bearing wear and subsequently resulting in lower rpm on the fan cooler). If active cooling is fine, and none of the passive cooling (heat sinks) have become dislodged, the give it a good cleaning to get all the dust and crusty stuff out and reassemble.
I blow out the dust and dirt out of my Shield every year or so. Like @Miki said, the disassembly and reassembly isn’t that hard as long as you have prepared the right environment to perform this so that you don’t lose parts. Dissipate body static before you start digging around.
If thermal thresholds aren’t the issue, you should make sure you’re not close to capacity in your internal storage. Just like the OS on a desktop, if your hdd is near capacity, the OS doesn’t have room to breathe as it needs space to put temporary files and typical OS bloat. I keep my storage under 70%.
Most of the Shield OS software updates have been stable. There was one a 3-4 updates back that broke the Bluetooth to some capacity but got fixed. I wouldn’t think it’s update related.
Too many apps going in and out eventually causes residual bloat that the android OS inevitably starts to slow and over longer periods of time, can slow to a crawl and apps get laggy, even shortly after a device reboot. The solution of this is a factory reset and reinstall relevant apps from scratch. Daunting, yes…but a necessary evil with many computer like devices that rely on an open OS environment.
I used to do a factory reset ever 2 years or whenever I felt OS bloat affected performance enough to bite the bullet earlier
Hardware issues can never be ruled out, such as a compromised memory module (ram dimms go bad over time, partially and entirely) or the onboard flash memory (eMMC flash memory) used for storage can get corrupted (file or directory structure can break) over time. These flash modules (like sd cards) have a finite number of write cycles and over years of use, these memory modules can wear out when the write thresholds are close.
The shield is built well with quality components. It’s generally durable. Don’t give up - just troubleshoot one area at a time and retest. My bet is that if it’s not a heat issue, a factory reset and fresh app install will fix things up nicely.
