Speed range not working

Hello:
I have 4 201 gecko drives, and I am having problems with 3 of them.
These 3 drives work fine from 0 to about 23,733Hz, then from there
up to 32,000Hz, it jerks up a little, it starts vibrating and the
shaft of the stepper motor sometimes locks, im sure its loosing
steps, from 32,000hz to 45,000 Hz it starts working fine again. The
other drive works fine from 0Hz to 45,000 Hz. Thats how I know these
3 geckos are not working properly. I have swapped steppers, port
pins and everything seems to point that the geckos are not working
properly. I am working with mach 2 version 6.00 with a P4 2.6Ghz PC.
Can anyone tell what is happening?
Best regards
Fernando Mayrl