iBook G3 repeatedly shuts down during use

Hello,

I installed NetBSD 9.0 rc2 some days ago, and I'm having a problem with 
random shutdowns during normal usage. This never happened on Linux or 
OpenBSD. I have not used any NetBSD release lower than 9.0, so I'm not 
sure if this is happening in other versions.

Randomly, even when idle, the computer will display the following warning:

	*** FINAL System shutdown message from root@iBookG3 ***
	System going down IMMEDIATELY
	
	/etc/powerd/scripts//sensor_temperature: CRITICAL TEMPERATURE! 	
	SHUTTING DOWN.

And the computer will turn itself off. A quick look at /var/log/messages 
identifies the problem as l_temp being too hot:

	Feb  7 20:46:03 iBookG3 root: /etc/powerd/scripts	
	//sensor_temperature: (dbcool0) critical
	limit exceeded [l_temp]
	Feb  7 20:46:14 iBookG3 syslogd[202]: Exiting on signal 15
	Feb  7 20:55:19 iBookG3 syslogd[178]: restart

These are the temperatures when the computer is idle:

iBookG3# envstat
                           Current  CritMax  WarnMax  WarnMin  CritMin  Unit
[AC Adaptor]
              connected:      TRUE
[cpu0]
               CPU Temp:    13.000                                      degC
[dbcool0]
                 l_temp:    59.250   60.000                      0.000  degC
                r1_temp:    55.250   80.000                      0.000 degC
                   fan1:     22500                                    RPM

So l_temp seems to be 59.250 while doing absolutely nothing, the maximum 
being 60.000.

My computer is an iBook G3, and the CPU is a PowerPC 750FX. This never 
happened to me, and it should be noted that on Linux the CPU temperature 
was always listed as "20 °C (uncallibrated)", even under heavy 
compilations, so I always thought that the CPU didn't even have a 
temperature sensor.

Do you guys know what could be causing this?

Thank you very much!