How much Virtual Memory do you use? Some apps don't like VM!
Video playback requires a lot of resources. If your app is attempting to save & read from the scratch disk it could potentially be a problem with video. Try turning VM off & see if it makes a difference. You'll need to restart your comp for the change to take effect.

Keep in mind also that LW & Java for os9 & earlier were not especially memory efficient. Launching a video app whilst you're using VM & whilst LW is doing multiple things (such as writing to disk & using VM) could be where the issue is. Some apps also use their own VM.

You have an older mac. The older HDD's were quite slow although your's would be a scsi. Add VM to that & everything gets slower. Also the system bus a bit slow (half the speed of the 1st G4) as is everything else video card, ram, etc. LW will make it work hard but perhaps the video launching is just a bit more than it can cope with, at least with VM being used.

Tell us if it makes a difference turning VM off.