yes marlonjohn that was probly the thermal throttling

i run a AMD athlon 3800+ dual core
it runs at 35 degC in normal use and max 55 degC in heavy rendering

i have a £10 case so dont worry about that
be clever where u put ur fans - i use the heatsink and cpu fan that came with the proccessor

have ur fans that are a the bottom of the case inserting air
at the top they should be extracting
hot air rises extracting it will cool down. as it is being extracted the cool air at the bottom will be drawn up (pressure diff) get warm then be extracted and it works a treat
i have 2 fans that is it

thats my opinion