I found out that our test-table in MySQL (yes..) used a smallint for our field ID.
As ID's where bigger than 32768, it was obvious it would stop..
Bad programming (not echoing mysql_error() cause a global wasn't defined) made it impossible to see.
Now I just made a restore of 1 table with more than 27.000 records and more than 9.0 MB big in just 6 seconds on a live site...
Thanks for the advise, cause I found out that table-caching only applies for SELECT/JOIN/SORT and stuff, but not for DROP/CREATE/INSERT (and probably ALTER/UPDATE too)
Erik