MacBookAir kernel_task at 300% and rising
Could you have a look at the 2 main system logs to see what your MacOS may be busy at.
With a terminal or an xterm window, the 2 following commands fired when the
problem appears may help you diagnose it:

tail -f /var/log/kernel.log

tail -f /var/log/system.log



To help anyone help you, next time this kind of problem arises, could you run the following command (within a terminal or an xterm window again):

top -o cpu


and grab a copy of the topmost 10 lines to put them back within your original question as here:

Processes: 50 total, 3 running, 47 sleeping, 243 threads                                                                                                                                       12:18:07
Load Avg: 1.40, 1.53, 1.28  CPU usage: 5.97% user, 7.14% sys, 86.88% idle    SharedLibs: 5044K resident, 9292K data, 0B linkedit. MemRegions: 10733 total, 769M resident, 15M private, 1479M shared.
PhysMem: 755M wired, 1363M active, 641M inactive, 2760M used, 1207M free. VM: 120G vsize, 1041M framework vsize, 2798981(19) pageins, 173(0) pageouts.
Networks: packets: 61207/73M in, 66123/6062K out. Disks: 877183/26G read, 171220/22G written.

PID   COMMAND      %CPU  TIME     #TH  #WQ  #POR #MREG RPRVT  RSHRD  RSIZE  VPRVT  VSIZE  PGRP PPID STATE    UID  FAULTS    COW    MSGSENT   MSGRECV   SYSBSD    SYSMACH   CSW      PAGEINS
1530  mdworker     17.1  06:56.12 4/1  2    82+  384+  39M+   16M    143M+  196M+  2586M+ 1530 1    running  501  1946213+  727+   881948+   219072+   6484219+  676187+   679401+  1914+
0-    kernel_task  14.0  07:55.29 69/4 0    2    757-  22M    0B     130M-  75M    2355M- 0    0    running  0    26279     0      33251333+ 32358872+ 0         0         10243109 0
1522  mds          12.6  14:55.33 9    8    118- 862   361M+  6052K  239M+  1334M+ 3736M+ 1522 1    sleeping 0    13373455+ 1286   12149111+ 6697348+  16039295+ 6938357+  3331251+ 1819924+
[...]


This is an example of a problem (no I don't have the same problem ☺) where my Mac was overheating like a fool.
Here the 2 processes involved are mds and mdworker and are caused by Spotlight indexing an external drive of 500 Go of backups
which I just plugged on.
The 2 involved tasks regularly topped to 200% cpu alltogether. Spotlight is very inconspicuous about this behind the scene heavy business. It does just display a small point at the center of its looking glass within the menu bar.

When I finally clearly saw what was happening under the hood, I decided to let spotlight finish its job, and one hour later, the temperature, the processors and the fans returned to a normal life.

Finally, since your OS (Mountain Lion) is Lion or more recent, I advise you to read this great explanation on sysdiagnose and if you feel comfortable reading system logs, try it.