Did you make any changes to the system prior to these lock-ups starting?
In addition to memtest, I'd suggest running a check on your hard drive, or at the very least checking the SMART diagnostics on it. A couple bad sectors can cause similar symptoms to bad memory.
Have you cleaned out the vents on the laptop recently? Thermal events can cause lock-ups, too, but in that case I would expect a total shutdown more than anything else.
What wireless drivers are you using?
Assuming your hardware checks out okay then I would definitely try another kernel, not just newer but you might also try one of the non-a64 kernels. I run almost exclusively AMD processors, but have run non-a64 kernels more often than not.