SharePoint 2013 Search: crawler worked fine - then stuck on FULL CRAWL

SP2013 Enterprise Search Crawler has worked fine for months but a couple of days ago it was stuck on a FULL CRAWL and could not find any content - no change to the configuration.

No errors in crawl log. No problems recorded with the SQL servers (no failover etc).

ULS was full of "lost connection - reconnecting" on CSSFeeder for Crawler...

CPU (one core) was 100% busy during crawling/slow respons.

Tried following and testet Crawl after each task:

1. Reset Search Index
2. Restart in Windows services: SharePoint Server Search 15
3. Recycle IIS App Pool for portal
4. Restart IIS web site for portal
5. Stop and start SharePoint Service in Central Admin: "Search Host Controller Service"

No luck.

The following did the trick:

6. Reset Search Index again
7. Reduced server load for Search Service (from default Maximum) to (use SharePoint Management Shell as Administrator):

• Set-SPEnterpriseSearchService -PerformanceLevel PartlyReduced

8. Restart Windows service: SharePoint Server Search 15
9. Started FULL CRAWL - success...