HDMI config settings wacky

[Mobius]

Ok, I just discovered the wonderful world of "config.txt", thanks to my shiny new Pi User's Guide from Eben and Gareth. My Pi was not discovering the HDMI consistently so I found the solution in the book. The problem, however, is that I can't get the settings to work consistently. My monitor likes 1366 by 768 at 60Hz. If I set hdmi_group=2 and hdmi_mode=86 it should work. But, nooooo. What I get is a default of probably 640 by 480. If I set hdmi_mode=16 I do indeed get 1024 by 768. Higher resolutions than that seem to default back to the low resolution output. But wait, there's more. If I comment out hdmi_group and hdmi_mode and just set framebuffer_width and framebuffer_height I can get the desired 1366 by 768. Any ideas why hdmi_group and hdmi_mode don't want to play nice?

[Wheel_nut]

Below are some extracts from the sample config.txt
Firstly, Is your Monitor an HDMI or DVI-D monitor? If HDMI, it will support sound over HDMI.
## hdmi_drive
## chooses between HDMI and DVI modes
##
## Value Description
## -------------------------------------------------------------------------
## 1 Normal DVI mode (No sound)
## 2 Normal HDMI mode (Sound will be sent if supported and enabled)
##
hdmi_drive=2

Next, you need to set the HDMI type. I suspect that you need DMT
## hdmi_group
## Defines the HDMI type
##
## Value Description
## -------------------------------------------------------------------------
## 0 Use the preferred group reported by the edid (Default)
## 1 CEA
## 2 DMT
##
hdmi_group=2

If you are now using DMT, then the correct mode for 1360 x 768 60Hz is Mode 39 so you need:
hdmi_mode=1

.... unless somebody else knows better ...

[Mobius]

I have tried all of what you listed. My question is why the Pi (or maybe the Raspbian SW) does not output the proper resolution when commanded to do so with the HDMI group/mode commands.

[Wheel_nut]

It does!

The Mode numbers are different in the CEA and DMT Groups and I believe that you are using the wrong modes for your Group selection.