What size are your jpegs? Very large still images don't perform well in video applications. Try not to make them a lot bigger than the resolution of your timeline.
Also, make sure they are RGB, not CMYK.
If these things don't make sense to you, let us know and me or someone else will help you optimize your stills for video.
Thanks for the reply.
The stills that seem to give me the most problem are the transparent .pngs.
They're grayscale (not RGB or CMYK)
and they're about 2x as big as my 1080p timeline. (I wanted to be able to slowly zoom in on some of these images etc without losing resolution)
Try making the stills RGB to see if that works.
2 times larger shouldn't be too much of an issue, but yeah, the grayscale is probably causing some grief.
If, after changing to RGB it's still hard, try reducing some clips to about 1 and a half times and see if this makes any difference.