Tixati V1.99 works fine with my normal internet connection, with download speed on ADSL2+ of around 1.1MB/s maximum, but I have just signed up with IPVanish to use a VPN.  Regardless of which server I select, my download speed climbs to around 350-400KB/s, and then drops to zero after about 90 seconds.  It then climbs again, and then drops to zero again. This is constant throughout the download process and is frustrating the life out of me.  What settings do I need to stop this from happening?

I would suggest you ask that question of your VPN service. Are you sure they allow torrent traffic?

There's no issue with torrent traffic through my ISP.  I've since raised the question with IPVanish, and they say:  "As with all communication that uses our service, it needs to be compatible with NAT (network Address Translation) since we utilize shared IP space to increase privacy. Some applications have something called NAT Compatibility (NAT-PMP port mapping) which you can enable and allows it to have repeated 2-way communication through the service. If your application does not have that, then you may want to find and download one that does. Personally I'd recommend utorrent."

I would prefer to use Tixati (ad free and more user friendly), if I can learn how to enable NAT-PMP port mapping, hence my question.

That is interesting. You may want to look at your router logs and the tixati logs look for a specific repeated error. From a quick search there seem to be many work arounds. It is not a very well thought of protocol so it appears a lot of apps don't support it but can still be made to function correctly using UPnP.

The number one thing that screws up most is a IPv6 address gets inserted where an IPv4 address is expected. Seems the common fix for a lot of problems is just to disable IPv6 on your router and network settings. Turning these settings off does not prevent you from using IPv6 but you should still check with your ISP about IPv6 settings.

I've been on IPvanish now for a long time, and have no problems of any kind, and change servers auto every 2 hours. Steady 6mb/sec download (my max speed) regardless of where in the world it connects.

I'm having same problem as new user of IPvanish. Tixati download use to be super fast using surfbouncer vpn and now very very slow using IP Vanish regardless of server/country choice. I did a Regedit in windows 10 to disable ipv6.