... there is no pattern rule for ending connections, it's random and under traffic usage.
I have setup-ed a PPTP server on MT v3.22. Created profiles for the connections that do not use encryption and no commpression. Have connected a couple of Linksys WRT54GL routers with DD-WRT OS installed with the v24 VPN.
The problem is when there is traffic passing trough, when the RDP session under the VPN is used the pptp connections get disconnected.
I have attached the lof sshot. As you can see the connection comes to life pretty soon, but I am interested every hour or so this happens, is there some settings for idle disconnection maybe or somethin other.
Any help would be appreciated.

The type of traffic should not affect this, unless there is something wrong with DD-WRT. I would suggest dropping in a MT at the remote site and seeing if it does this too. I have extensive experience with this in business and corporate environments and I know it does work quite well. Without taking a look at it, I couldn't tell you too much more. I am sure a MT consultant can help you out.

I have 3 routers WRT54GL with the latest DD-WRT. Two routers have ADSL connection with 768Kb/64k speed. Third one has 512Kb/128Kb cable connection. Two routers I use for RDP session with the Central office where MT v3.22 is located with 3Mb/256Kb ADSL link, for an application that spools reports at the Win2k3 TS that is located in the CO.
I have noticed that the third router with the ADSL connection that I have setup-ed at home has 24hrs of PPPT connection UPTIME with no session endups, I do not use an active RDP session, but the two other routers connected to the CO after a while in the log says: pptp-out1 logged out followed by numbers, and in a couple of seconds the device authenticates again and logs on. I do not need any encryption for the traffic, just stable connection.

This the log from syslog of DD-WRT
23:02:11 pptp[4672]: anon log[logecho:pptp_ctrl.c:676]: Echo Reply received.
23:01:11 pppd[4669]: pppd 2.4.4 started by root, uid 0
23:01:11 pptp[4672]: anon log[ctrlp_disp:pptp_ctrl.c:896]: Outgoing call established (call ID 0, peer's call ID 42).
23:01:11 pptp[4672]: anon log[ctrlp_disp:pptp_ctrl.c:857]: Received Outgoing Call Reply.
23:01:11 pptp[4672]: anon log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 7 'Outgoing-Call-Request'
23:01:10 pptp[4672]: anon log[ctrlp_disp:pptp_ctrl.c:772]: Client connection established
23:01:10 pptp[4672]: anon log[ctrlp_disp:pptp_ctrl.c:738]: Received Start ontrol Connection Reply
23:01:10 pptp[4672]: anon log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 1 'Start-Control-Connection-Request'
23:01:10 pptp[4669]: anon log[main:pptp.c:267]: The synchronous pptp option is NOT activated
23:01:05 pptp[4612]: anon warn[decaps_hdlc:pptp_gre.c:209]: pppd may have shutdown, see pppd log

Maybe if I determine the optimum MTU setting for the connection this problem will stop.
I have pinged the external IP address of the mikrotik which is using ADSL modem and acting as a PPTP server.
No I have setup-ed the MTU to the default 1460 value and will see some time if the connections will drop.
U used command ping xxx.xxx.xxx.xx -f -l xxxx (as packet size) on the external IP of the MT, and the packet bigger than 1452 was returning fragment message.

Any help would be appreciated.

I have setup MTU of 1440. I have tested the random VPN disconnect. I have setup-ed a Wrt4gl DD-wrt at my home, and I have noticed that the vpn breaks when I use the whole bandwith, say I am downloading a torrent file and use max down and up so the vpn logs out. A the remote locations how can I prevent the bandwith usage or something else to make the vpn connctin stable.