Vim: Automatically leave insert mode after some time - vim

Question

I want to make Vim leave Insert mode automagically after some time (15 seconds). Here is what I've got in
my .vimrc:
" set 'updatetime' to 15 seconds when in insert mode
au InsertEnter * let updaterestore = &updatetime | set updatetime=15000
au InsertLeave * let &updatetime = updaterestore
" automatically leave insert mode after 'updatetime' milliseconds of inaction
au CursorHoldI * stopinsert
I am using this snippet in MacVim, and it doesn't work. When I enter Insert mode, and I don't do anything,
Vim leaves insert mode after 15 seconds. However, if I press Enter or Backspace, or if I enter Insert mode
using o/O, then Vim leaves Insert mode straight away, and at that point, even if I enter Insert mode using i
Vim goes back to Normal mode straight away.
I installed MacVim using Homebrew on Mac OS X 10.8.5
Any ideas why this is not working please?
Edit: Here is my MacVim version with all the flags:
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Sep 29 2013 02:17:27)
MacOS X (unix) version
Included patches: 1-22
Compiled by Homebrew
Huge version with MacVim GUI. Features included (+) or not (-):
+acl +file_in_path +mouse_sgr +tag_binary
+arabic +find_in_path -mouse_sysmouse +tag_old_static
+autocmd +float +mouse_urxvt -tag_any_white
+balloon_eval +folding +mouse_xterm +tcl
+browse -footer +multi_byte +terminfo
++builtin_terms +fork() +multi_lang +termresponse
+byte_offset +fullscreen -mzscheme +textobjects
+cindent -gettext +netbeans_intg +title
+clientserver -hangul_input +odbeditor +toolbar
+clipboard +iconv +path_extra +transparency
+cmdline_compl +insert_expand +perl +user_commands
+cmdline_hist +jumplist +persistent_undo +vertsplit
+cmdline_info +keymap +postscript +virtualedit
+comments +langmap +printer +visual
+conceal +libcall +profile +visualextra
+cryptv +linebreak +python +viminfo
+cscope +lispindent -python3 +vreplace
+cursorbind +listcmds +quickfix +wildignore
+cursorshape +localmap +reltime +wildmenu
+dialog_con_gui -lua +rightleft +windows
+diff +menu +ruby +writebackup
+digraphs +mksession +scrollbind -X11
+dnd +modify_fname +signs -xfontset
-ebcdic +mouse +smartindent +xim
+emacs_tags +mouseshape -sniff -xsmp
+eval +mouse_dec +startuptime -xterm_clipboard
+ex_extra -mouse_gpm +statusline -xterm_save
+extra_search -mouse_jsbterm -sun_workshop -xpm
+farsi +mouse_netterm +syntax
system vimrc file: "$VIM/vimrc"
user vimrc file: "$HOME/.vimrc"
2nd user vimrc file: "~/.vim/vimrc"
user exrc file: "$HOME/.exrc"
system gvimrc file: "$VIM/gvimrc"
user gvimrc file: "$HOME/.gvimrc"
2nd user gvimrc file: "~/.vim/gvimrc"
system menu file: "$VIMRUNTIME/menu.vim"
fall-back for $VIM: "/Applications/MacVim.app/Contents/Resources/vim"
Compilation: cc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_MACVIM -Wall -Wno-unknown-pragmas -pipe -DMACOS_X_UNIX -no-cpp-precomp -g -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -I/System/Library/Frameworks/Tcl.framework/Headers -D_REENTRANT=1 -D_THREAD_SAFE=1 -D_DARWIN_C_SOURCE=1
Linking: cc -L. -L. -L/usr/local/lib -o Vim -framework Cocoa -framework Carbon -lncurses -liconv -framework Cocoa -fstack-protector -L/usr/local/lib -L/System/Library/Perl/5.12/darwin-thread-multi-2level/CORE -lperl -lm -lutil -lc -framework Python -F/System/Library/Frameworks -framework Tcl -framework CoreFoundation -framework Ruby

Answer 1

As #romainl commented, the fastest way to narrow down a problem like this is bisection. Basically, you disable (comment out) half of your vimrc to see if you still have the problem, then test it with the other half. If only one of those halves has the problem, you split it in half and repeat until you find the offending setting/plugin/etc.
For sanity's sake, with an issue like this it's also a good idea to start by testing a minimal setup (i.e. a vimrc with just the lines quoted in the problem description) to make sure it's not a wild goose chase.