That camera of yours records videos using the QuickTime (.mov) container with the H264 codec (an MPEG-4 variant).
This codec is not meant for an editing application like FCE, as not every frame is stored, something FCE and other editing programs need to properly edit the footage.
In order for you to edit those videos without constant rendering, you need to convert H264 .mov files via MPEG Streamclip to .mov files using the Apple Intermediate Codec (AIC), which you then can import into FCE and use without rendering.
You have to know, that the footage in the FCE bin and the sequence settings have to match, and as FCE does not have an H264 sequence setting (if you create one, you will create just more trouble as seen
here), you have to use one of the Apple Intermediate Codec settings.
MPEG Streamclip has a Batch function (CMD+B), so you don't need to convert every single file by hand.
Also make sure you have the proper settings for the conversion, as far as I gather from the Specifications page from
Canon's official representation of the SD 780, your footage needs to have a resolution of 1280 x 720 (720p) and a frame rate of 30fps (frames per second).
Be sure to also read the manual of FCE (Menu Bar > Help > Manual or look
here).
PS: The constant rendering is due to the differing of the footage and the sequence in its properties, using a different codec mainly. The rendering solves that problem temporarily by creating AIC encoded footage for proper viewing, but every change to a clip in the timeline, being trimming it or moving it, results in another rendering of said clip.