my 2600+ did the exact same thing, but it wasn't the case coolings fault. i got this
HSF from newegg for 13.99 along with some as5 and it dropped me from 52c idle to 44c and from 64c load to 52. (that was before my as5 cured all the way) then i bought this
fan to go on it.... basically it made it like a volcano 12, for about the same price. (got money after i already had the HSF, so i just upgraded..) altogether i now idle at 35.5c and load at 40c, so down about 17c idle and 24c load for about $30. thats the best cheap HSF i could find, notice that its all copper, very nice. so if all those case fans dont help, you might try out this.