I just went through this with my Microstream. They had sent me a new cap a while ago and I believe they sent two. Fixed it for a little while. Now my light doesn't work at all. Contacted them again and this was there reply. Looks like I have to send it in now.
Quoted from streamlight below
Please perform the following test to see if it could possibly be your switch
1.Remove tail cap.
2. Touch the back of the batteries with a flat head screwdriver
3. While keeping contact with the batteries,
move the screwdriver horizontally until it makes contact with the barrel.
4. Keep contact for a few moments to ensure the light stays on.
If the light fails to light or shuts off the problem is with the LED in your light which is covered under warranty. Please send your light here to the factory for warranty repair/replacement using our online service request form
http://eweb.streamlight.com/Streamlight_Service/default.aspx
If the LED lights and stays lit, the problem is most likely with your switch. In that case, please reply to this email with the color of your tail cap and your mailing address. We will mail a replacement tail cap as an accommodation.