I've read that this is caused by either your ISP, your router, usage of NAT on your router, your OS, or firewall ports on the corporate office blocking some ports that are using the PPTP VPN process. With all these variables, we definitely need a bit more information.
Are you the only one experiencing this?
How do you connect to the Internet?
Some of these can be easily isolated, whereas others can't.
RRAS does log detailed information in C:\Windows\tracing, and standard logging on C:\Windows\System32\Logfiles on the RRAS server itself or the RADIUS server (depending on your setup). Maybe the logs there can help.