The back of the ignition switch, which has the electrical part is notorious for causing these problems. I had this type of problem and just changed the back electrical part - you don't need to remove much to do it, from memory - just the covers on the steering column to get to the back side of the ignition switch, if memory serves me correctly. If you buy a replacement, I think there are some slight differences in the electrical plug from memory. So it is probably a good idea to have a look at your own, before you lay out the money. I think there are a few threads on this forum about it, which should provide you with the info you need.
cheers, Tim.