PPTP VPN Connections drop more than expected.

Hi Guys

Just wanted to know some general insight, i have a Mikrotik that has about 100 VPN connection via PPTP, but the connections drop all the time.
Is there some setting I can enable to keep the tunnels from dropping?

I can understand that some of the connection drops are legit, i.e broadband connection disconnections etc. But i have sites that do not loose broadband but still the PPTP VPN connection disconnects.

I am using windows and the standard windows dialer for PPTP connections.
I have played around with keep alive which is default at 60 seconds but changing this to 5 or 120 seconds has had no effect.

Any info would be very much appreciated