After doing a lot of research (in wrong direction) I figured out that my Linux Mint distro, Wireless adapter, remote PPP Windows server, ISP were not guilty for PPTP dropping connection. Culprit was my router D-LINK DIR-300 firmware revision B1, and whatever setting I applied nothing was helping – PPTP VPN connection would just drop after minute or two or even immediately. I got confused by my Windows laptop having perfect VPN over home Wifi while my Linux Mint desktop with Wireless adapter was dropping connection, I even installed windows drivers, changed MTU for all connections etc. Finally I decided to test my router and after few minutes I was sure it was dropping my VPN connections (the other day I had same problem with inbound VPN from my work).
DIR-300 is very good router, I have port forwarding, SSH and many other services working perfectly but VPN was just beyond repair. Out of despair I decided to change factory firmware to DD WRT.
I tried to follow these instructions but I wasn’t able to connect to WAN port through Telnet with Putty and access RedBoot prompt. Instead simplest solution worked for me. Just go to DD WRT website and use Router Database to find your router – in my case D-Link DIR-300 B1. Download “dlink-dir300b-factory-webflash.bin” (not the other file). Enter your router administration console and reset to factory defaults, login again and update firmware with this file. After that (2 minutes) you will be able to login to DD WRT (password is already entered for you, change it).
Last step gave me shiver, it was time to test VPN stability. Success! Everything is working fine now. Remember to always check your network equipment as much as you can before blaming software or operating system.