Same Query Takes 1 Sec Or 5 Minutes!

I am truly stumped. I have a query that sometimes takes less than 1 second to run, and sometimes takes close to 5 minutes. I am making NO changes to the query. I first thought it was a network issue, so I moved the whole db, both front-end and back-end, offline. No change. So I thought maybe it was a linked table issue, so I moved all the tables used by the query to the front end. No change. Every once in a while, the query runs at lightning speed, less than 1 second. Most of the time, it takes 3-4 minutes, or longer.

Here's the SQL:
SELECT PCAInternalPareto1a.row, PCAInternalPareto1a.column, PCAInternalPareto1a.Noun, PCAInternalPareto1a.Rev, PCAInternalPareto1a.InspPoint, PCAInternalPareto1a.cell, PCAInternalPareto1a.DefectType, CLng(nz([firstofTruncatedOpSeq],0)) AS ZOpseq, Sum(PCAInternalPareto1a.DefectQty) AS SumOfDefectQty, PCAInternalPareto1a.poo, PCAInternalPareto1a.repcode
FROM PCAInternalPareto1a LEFT JOIN BOMOutRefDesOnly ON (PCAInternalPareto1a.RefDes = BOMOutRefDesOnly.RefDesOnly) AND (PCAInternalPareto1a.partno = BOMOutRefDesOnly.PCAItemNo)
WHERE (((nz([FirstOfComponentItemNo],0)) Like "*"))
GROUP BY PCAInternalPareto1a.row, PCAInternalPareto1a.column, PCAInternalPareto1a.Noun, PCAInternalPareto1a.Rev, PCAInternalPareto1a.InspPoint, PCAInternalPareto1a.cell, PCAInternalPareto1a.DefectType, CLng(nz([firstofTruncatedOpSeq],0)), PCAInternalPareto1a.poo, PCAInternalPareto1a.repcode;

PCAINternalPareto1a is itself a query, which always runs fast. BOMOutRefDesOnly is a table with about 84,000 records. I have indexes on the joined fields.

I know the where statement looks useless but I have my reasons for doing it this way. I tried deleting the where statement and it ran really fast. Then I re-added it, and it still ran really fast. THen slow again. I can't find any relationship between anything I've tried and the speed of the query!!!! What's going on??????

Access 2002