Ethernet stalls intermittently


I have a Diecimila with an ethernet shield and I'm having some rather odd issues that I'm fairly certain are due to the power supply issues. The board acts as a server that, when queried, returns the values of all the analog inputs, 3 of which are connected to resistance based temperature sensors feeding from the 5V supply and the other 3 are left floating (maybe part of the problem?). When starting out I found that any power surges like my A/C turning on or my dryer starting, would cause the board to crash. In trying to change out power supplies (9v wall warts) I had to use an extension cord and found that for some reason the extension cord seemed to act as a filter and prevented the board from crashing at the drop of a hat. Now that the board wasn't crashing every 5 minutes, I wrote a python script to poll the board twice per second keeping track of how many times it got a successful reply. The board ran for 1,835,863 queries (about 7 days) but then stalled for no apparent reason. I have repeated this several times and the board runs from 300 to a million times before crashing, which make me think that there are still some power issues that need to be resolved. Which brings me to my actual questions:

1) Should I build an additional power supply to go in between the wall wart and the Arduino?

2) Would it be more effective and fault tolerant just to use the watchdog timer?

3) Am I crazy to think that the extension cord is acting as a filter due to increased induction?