I’ve checked the JOGL demo and it also happens (with and without DirectDraw enabled). Even, like I said before, using no Thread.sleep but only thread.yield (and consuming 100% of CPU) the gaps happen.
Wonderful!! Slick works perfect (windows or full), any idea?
no much idea, maybe a lock issue ? are you using intensive synchronized blocks ?
EDIT : also a full ( but short ) test case will be usefull, do you have any ?
Yes, see few topics about the same issue. Maybe gives you some ideas to try.
see this smooth scroll topic
simple game loop
FIXEDDDDDDDDDDD!!! 
it was a problem of having VMWare installed (but not running), if I close the process vmw-auth (or something similar) even if it is not consuming any CPU ticks, the problem gets solved.
What is more weird, this strange and undesirable effect, affects only the applets and jnlp applications (only slick applications work fine if they are jnlp or applet).
Loads of thanks to all for your reply.
: Maybe send a bug report to those bastards