Well since you have a new computer and you tried different serial cables and he rain gauge is now showing erratic readings I would guess the 3600 console took a hit from the thunderstorm you mentioned. You verified that the computer is setup correctly with the com port? 9600 baud, 8 bits, parity none, stop bits 1, no flow cntrl, etc. and it's using the correct port that you connect to? On the computer if you look in "my computer", "control panel", "system", "hardware", "device manager", does the listing for communications port have a yellow exclamation point symbol next to it indicating a conflict problem?
Regards
Dan