I think it comes down to something basic. When people post replies saying you should search out, they generally feel when you find those related threads, it will provide all the answers you need. In that case there's no need to reawaken or start a new thread. Though, if the found threads don't supply the answer or don't resolve the issue, then you have to determine the best way forward.
If the old thread is very similar and isn't very long (less than 15-ish) then I'd say it's fine to awaken it (but realize some peoples won't realize how old the thread is and thus may also overlook your post). You may want to add the comment at the start of your post that you realize it's an older thread. Also, be sure to note in your post what in the original thread pertains to your situation. So many people simply say, "I have the same problem. Please help!!!!" and that isn't very helpful at all. Remember, you're the one wanting help, so make it easier for others to help you.
If an old thread is similar, but you have some more specific issues on hand or whatever, then I would supply links to those threads just to show that you've been searching around and you can also note how the problem in that thread is similar and different from your own problem. Properly done, this can get your issues fixed more quickly. Doing this does take more effort, but by showing you're willing to do some of the work on your problem, people are more willing to help and it often helps keep the trolls a little farther away.
