inconsistency errors and how to fix them?

One of the db's I support is prone to throwing up inconsistency errors from time to time. The latest problem seems to relate to indexes.

Server: Msg 2511, Level 16, State 1, Line 1
Table error: Object ID 645577338, Index ID 3. Keys out of order on page (1:286818), slots 308 and 309.
Server: Msg 2511, Level 16, State 1, Line 1
Table error: Object ID 645577338, Index ID 3. Keys out of order on page (1:286826), slots 308 and 309.
DBCC results for 'OrderItem'.
There are 9973753 rows in 67394 pages for object 'OrderItem'.
CHECKTABLE found 0 allocation errors and 2 consistency errors in table 'OrderItem' (object ID 645577338).
repair_rebuild is the minimum repair level for the errors found by DBCC CHECKTABLE (Iceland.dbo.OrderItem ).

I could run a DBCC checktable with the options that *might* fix the above. I am also thinking about doing an index rebuild (currently a nightly index defrag is done but no schedule for an index rebuild is in place). Another table is involved. Both table are reasonably large (one is 7 million rows and the other is nearly 10 million rows). These tables are involved in a chain of many to many relationships. I don't know if it's a factor but the chain is circular. However, this doesn't seem to cause problem in other dbs that have the same design.

What would be the recommended approach for:-

a) fixing the current db inconsistencies?
b) finding the underlying cause of the problem to stop it from happening again?

Thank in advance,

CliveMicrosoft Knowledge Base Article - 822747