my bulb went wierd before and it was corrosion in the bulb plug had gotten hot / rusty and eventually damaged the plug. Though i also had a correded fuse connector aswell....
Id personally start from the light switch and work my way into the engine bay...there are relays for the lights in the engine bay, maybe you've got a bad connection or something somewhere in that lot. Sorry i cant give you a "check here, this is your fault" kinda answer.
How good you are at reading wiring diagrams? 
as the haynes is fairly good at pointing you to the wiring youd want to check.
