Sounds to me like there is a partial break in the perimeter wire or a break in the wire's insulation. Rain is causing the signal to get shorted to ground.
If the lights on the transmitter are flashing normally, it will be difficult to find the break. If the light is steady green, it means it is actually a full break and the signal is transmitter over the air at the break.
We have now listed a Perimeter Wire Break/Cut Finding Tool on our site. It comes with special instructions for robotic lawn mowers. The tool can be used to find a full break in the wire.
Finding a partial break is harder, unless it happened at a splice (most likely) and you know where the splices are. If you know of places where digging occurred in the past, that is also a good place to start.
If you are desperate to find a break after ruling out all of the easy places it could be, then what you do is deliberately splice the wire in half. See if the robot starts up now. If it does not, you know the break is in that half so it reduces the amount of wire inspection you have to do.
If it does recover, you know the break is in the other half. Move the base to that half using an extension cord to the electronics. See if the robot has no signal again. This confirms the break is in this half of the wire. Now go find the break, or repeat the process if you want to narrow it down some more.
At some point of reducing the length of the wire, the robot may recover because the shorter length may allow enough signal to be present. If this occurs, you might try moving the robot to different places in the reduced loop to see if it works in some places but not in others.
