I haven't noticed that there was already fixed version when I've posted this. Thanks Carciofone.

I've tried your method and it does not quite wants to work.
Added restart_wan (not sure if it's needed but I've also set 'x' permission to it) to /tmp/cron.d/ with that content:
Code:
00 15 * * * killall pppd
and connection is still happily working at 15:01
Code:
Current Time: Mon 11 , Jun 2007 15:01:18
Connection uptime: 0 Days, 2 Hours, 13 Minutes, 1 Second