100nsec per div and faster sounds like the switch from real time sampling to equivalent time interpolation (300Msample/sec real time). You can turn off the equivalent time mode by going to the Acquire menu and setting 'Repet' to Off. If that enables you to get to faster sweep speeds then the problem is clearly in the circuitry that does the equivalent time interpolation but I would need a thorough read of the manual to go into any detail!
Regards,
Roger
