Index causes query to break

Hopefully this won't be too hard to follow. Please let me know if I need 
to clarify something.

I have two types of Nodes (let's call them LizsNode and LizsNodeDesc) 
that each have an attribute named ID. LizsNode uses a namespace (tns).

I have had the LizsNode data around for months, but the LizNodeDesc are 
all new. I have added an index defined as "*@ID" recently.

With the index defined, when use the xpath query 
"/DatabaseRecord[//tns:LizsNode[@ID='1234']]" the node is returned 
correctly. But when I use the query 
"/DatabaseRecord[//LizsNodeDesc[@ID='1234']]" 99.999% of the time 
nothing is returned. Occasionally the right result is returned. Also if 
I query for /DatabaseRecord[//LizsNodeDesc]" all those nodes are 
returned and I can see my ID is correct.

If I remove the index all queries return correctly, but its way too slow.

Also, I have one machine that this seemed to occur on but I deleted the 
entire collection and the newly created one is fine. I cannot do this to 
my production server however.

Any ideas what is causing this? I have confirmed the issue by querying 
via commandline and via Java code.

Liz