idles fine. while driving it will sputter, pop out the exhaust etc while making a "bra..bra... braaaaaah" sound. it does this intermittently while driving and only when its raining or snowing (guess what is going on outside) if there is a dry day, it doesnt do it.

any ideas as to what is going on?

Have you checked you distributor cap for cracks? Next time it's raining spray some WD-40 inside the distributor cap and see how it runs. When was the last time you replaced the plug wires? I'm thinking it's somewhere in the ignition side.

another simple thing to check is to see if the passenger side floor board is rusted through--causing moisture to get to the computer behind the passenger kick panel.

Also check all your spark plug wires, especially for arcing. Plug wires can short out when wet. The easy way to tell this is to first start the vehicle in a dark garage or at night outside and look for any light shows under the hood. If you don't see any, shut the engine off and use a spray bottle and lightly mist the plug wires, distributor cap, and coil wire, restart, and see if you see any light shows then.

Also check the electrical connections at the coil and make sure they are snug.

Are your front wheel well splash guards in place? How about the front skid plate (if '86 or newer) or the splash pan if '85 and older?

Also try spraying the O2 sensor leads.

If you need to replace the O-ring that goes between the cap and the distributor, it's $2.35 from www.1sttoyotaparts.com, part number 1912715021 (dust packing or some thing)