I'm using Tixati and am always using a VPN with a kill switch.  I've never received notice from my internet provider (Spectrum) that I'm in breach of their terms of use.  But using Tixati and VPN at home gives very spotty i.e. fast/slow/zero downloading, and I also note that my VPN gets booted off on occasion and I see it re-starting.

I have no apparent issues with my VPN with any other applications.  I have no problem downloading from Usenet (for example) in a fast and consistent fashion.  I assume that the problem that I've described is NOT intrinsically related to Tixati, since it works flawlessly away from home.

Is this likely related to my IP provider?  Or is there some setting that I can tweak to improve things?  Incidentally, I have the maximum download speed set at 999999.

First of all, make sure Tixati is not allowed to connect without the VPN connection: Settings > Connections > Local IP address or interface

It is possible your ISP detects Bittorrent/VPN or BT+VPN (now that would be worrisome) traffic and throttles it. Before you suspect the worst is it possible you are leaking DNS requests to your router instead of sending them to the VPN server?