How's the alternator? I'm guessing that might be bad. I've had the exact same thing happen to me before. Driving in very heavy rain, car bogs down, speedo goes up and down (which is typical of an electrical failure). If it is your alternator, you might be able to check battery voltage to see if it's dropped any. If you run the car long enough without recharging the battery you'll end up stranded somewhere. Have it check out at an auto shop. Check and make sure the belt is still on and in good condition.