Early on with MOSS I saw a lot of instability in the search and SSP functions. It sounds like Nick Hadlee had some similar experiences. I saw some issues where the indexer continued to lock up, and after some lengthy troubleshooting it was found that there was some corruption with the SSP. The SSP had to be recreated and then it started working again. This was all on SP1, long before SP2 was out and I haven't seen this since SP2 was released.
In addition to looking at the crawl logs, I would also check the ULS logs to see if there are timeouts or other errors relating to the indexing process. With that much content it might be a good idea to try adding a second web front as a dedicated web front end for crawling.