When you have more than 100ms/div selected and running in Standard Mode or Chart Recorder Mode, the max sample rate is 1KHz and probably much slower. Data streams to the PC and the ADC doesn't use it's internal memory to store data. During this time the PC gets quite busy and if it has other tasks to perform gaps in the data may occur and the PC may appear to lock up (depending on the PC)
Since you only have this issue at the longer time bases, I suggest you use Block Mode. This will make the ADC store the data in it's internal memory and transfer the information to the PC in blocks. There will be no gaps in the collected data and the PC does not affect it. This should solve the problem. You will also be able to run at full sample rates and collect many more samples for a better waveform resolution.
At the faster time bases, the mode has no real effect, but as soon as you put more than one second on the screen the mode is everything. Try it, you'll like it