38C is actually pretty good for a dual-core AFAIK (thought I read AMD's recommendations were about 50C?). Try having it sneak up to 60C (yes, and even idle) like mine was before I got fed up with the stock cooler and picked up a Freezer 64 Pro. Mine was also doing this consistently across any OS so that part may not apply.
I'm not saying you should live with the temperature spikes apparent with 10.5.6 but I am saying not to panic unless the temperatures worsen. You should probably set up a CPU warning in the BIOS for when it reaches 60C just in case.
Just my two cents regarding the thermal woes. Take it or leave it.

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Custom Hackintosh...AMD Athlon X2 5000+ (2.6 GHz) on GA-MA78GM-S2H...Linksys WMP300n Wireless...ALC889a sound...ATI HD 3650 PCIe...1 GB DDR2 RAM...320GB Western Digital SATA + 200GB Seagate IDE HDD...Rosewill DVD-RW IDE...Running Kalyway upgraded to 10.5.7...All Fully Functional