Cron jump to 500 internal server error while run indexing process

Hello everyone, this is my problem:

That's what I did:

1. I did a fresh install of Drupal 7.15.

2. Disable the "poors man Cron".

2.5. Import about 2000 nodes
from a previous installation, this installation was old Drupal 6.

3. Activate the module searches.

4. Set 100 nodes by indexing process.

5. Manually run the cron (admin / config / system / cron)

6. After a wait of several minutes, the error 500 server.

7. In admin / reports / dblog no record thoughts about the end of the execution of cron, appears not log any errors.

8. The rest of the site is working well.

9. Again, run the cron manually (admin / config / system / cron)

10. Mysteriously, appears in the log (admin / reports / dblog) the report "Running cron finished" with fixed end time in an hour before the present.

10. After a wait of several minutes, the error 500 server again, because of the process initiated in 9.

11. The site is indexed on the percentage expected after the first execution of cron.

12. No flag appears indicating the end of the second execution of cron.

I think that may be a problem with bad quotes embedded in the content of the imported nodes that are affecting indexing.

Hacerca What do you think of these problems?

What I can do?

Thanks