Sounds like you've got a bad thermal monitor. 34 won't crash a CPU (you were talking about CPU temp weren't you and not motherboard temp?)
At any rate, the Athlon spec calls for 85 degrees Celsius case temp on the newer Athlon XP's, 90 degrees on yours I think. But this is actual CPU case temp... many thermal probes are located under the CPU socket, so they will be reading lower. The Athlon XP I believe now has an on-die thermal diode (like Intel has had for a long time) and that should be a bit more accurate.
However, my experience with thermal monitors is they are fairly inaccurate... I pull a CPU along with it's heat sink and fan and put in in another motherboard and read temps 10 degrees different! My rigs start to crash about 55 degrees...
But if you're crashing, your obviously too hot. What kind of heat sink and fan do you have? Are you using thermal paste on it. Also, do you have case fans to keep the inside of the case air cool? All these need to be in place to run an Athlon properly. If you want to know what some good fans look like, browse over to www.coolerguys.com and look over their offerings of Socket A coolers.
Scout
700 Mflops in SETI!