Using the sigmon script, belonging to the gcom package, kills pppd
started with wvdial.

Steps to reproduce the problem in my system:

1) sigmon can be used to monitor signal strength. Nothing seems to be
   killed while pppd is not running.

2) wvdial should be configured. Here's the contents of my
   /etc/wvdial.conf:
	[Dialer Defaults]
	Init1 = ATZ
	Init2 = ATM1L1
	Init3 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
	Modem Type = Analog Modem
	ISDN = 0
	Phone = *99***1#
	Modem = /dev/modem
	Baud = 460800
	Username = ''
	Password = ''

	I symlink'ed /dev/modem to /dev/ttyUSB0.

3) Fire up wvdial. After a few seconds the connection is established.
   Doing a 'ps auxw' to see running processes reports that the pppd
   command line reads:

	/usr/sbin/pppd 460800 modem crtscts defaultroute usehostname \
	-detach user '' noipdefault call wvdial usepeerdns idle 0 logfd 6

4) At any time after this, fire up sigmon. It begins to probe the signal
   strength. pppd is killed as soon as the script is started and connection
   is lost.

[ ... ]
pppd was never terminated. Is there any helpful syslog output to find
out why pppd terminates? Did you try another kernel?

greets Jimmy

P.S.: Sorry for the late reply, was on vacation.