1080p24 transmits a lower bitrate than either 1080p30 or 1080p60. This suggests that there might be a problem with one or more of the HDMI cables (too long, or standard instead of high-speed design) causing excessive attenuation and/or dispersion of high frequencies. If you haven't already, you might try Monoprice's Redmere cables. They include electronics to improve the quality of the HDMI signal. They're also thinner and lighter, putting less strain on the connectors. External HDMI equalization boxes are available, too, but they tend to be more expensive. As a check, you might try lower computer screen resolutions: 1280x720 (720p) or 640x480 (480p). They're significantly lower bitrates. If they work, then it's even more likely to be an HDMI cable problem.