The internal diagnostics of the nav don't test memory, here's what they test:

NavCoder v2.5.40
Navigation diagnostic data retrieved 3/01/2009 00:01:35

Diagnostic command acknowledged

Wheel sensor test status: OK
Wheel sensor, Left: 0 pulses per second
Wheel sensor, Right: 0 pulses per second
Gyro test status: OK
Gyro voltage: 2445 mV
+5V logic supply test status: OK
+5V logic supply voltage: 5.076 V
+12V vehicle battery voltage test status: OK
+12V vehicle battery voltage: 12.184 V
Voltage sensor test status: OK
Voltage sensor voltage: 1.253 V
Cooling fan test status: OK
Cooling fan state: Off
Eject button test status: OK
Eject button state: Not pressed
Temperature test status: OK
System temperature: 23°C
Vehicle direction test status: OK
Vehicle direction state: Forwards
GPS test status: OK
GPS state: Reception error, check antenna

But check your nav when it crashes. Don't reset it - run the diagnostic and compare the results with above (normal) status.