I have a DSLR camera which can record a HD movie at 60fps. My question is that, sometimes when recording, the FPS seems to drop below 60fps. What caused this problem? I used a class 10/UHS-1 SD card with transfer rate up to 45MB/s.
You must remember that the bitrate of the compressed file does not represent the decompressed file size. Your PC is trying to reconstruct 60 full HD frames per second without a specific chip set to do it (whereas your camera is designed for this one task).
It is not at all surprising that it cannot achieve this, especially if there is not enough video ram allocated to displaying all the data. Many Video Editing applications will scale the dimensions or frame rate to retain audio sync playback. Consider creating a higher bitrate file that requires less processing time per frame but uses more memory to decompress. This should alleviate some of the stress on the CPU for decompressing.