Toyota Repair Problems? Ask a Mechanic for Answers ASAP
Hello and welcome to just answer. I value your input to my questions so I may help you better. This may be very difficult because it's intermittent. The one thing I would try doing is to try and duplicate the lights flickering by wiggling some wires as it runs. I would start with the wires coming from the alternator and it's connections. also around the fuse block in the engine compartment. Check all connections for corrosion and tightness from the battery.
Here is a description of that code which usually indicates a catalytic converter is bad. http://www.autocodes.com/p0420_toyota.html
How many miles are on your vehicle?