I had a similar problem with 555 led's particularly in the pop bumpers . The way I fixed it was to use a small blob of silastic between the led and the lamp holder to "stick" them in place . It's soft enough that if you need to remove the led later you can .
A couple of tips though :
- Don't get any silicon on the contacts (not sure how it would react)
- Make sure you put the 555 led into the lamp holder the right way before "gluing" it (most 555's are polar sensitive so will only work when installed the right way).
