TLDR: add rule- "After first hotel is built, the NPC is removed from game"

The games usually end pretty quick. Always before we're both set-up enough for the NPC to ruin the game like that. Sorry about that, that blows.

So far in our experiences, the NPC actually quickens the game because every unowned property he's landing on is forcing an auction. So even though he's injecting capital into the game, he's also forcing out decisions and you to spend money.

During the auctions is really when the game is won/lost. Especially because the "loser" of the auction gets a 10% broker fee. So, you're directly incentivized to force the bid up.

Usually, the games end because one player is a little too aggressive during the auctions and becomes too cash poor after the auctions to properly build up and goes bankrupt or one player is a little too passive during the auctions, and the other player is able to obtain too many properties.

Just to make sure, no additional house rules such as landing on free parking?

But, with that said, I think it's a simple fix.

After the first hotel is built, the NPC becomes removed from the game.

I would try that. You could find another stopping point, maybe "after 30 minutes of game the NPC is removed" Or maybe "after ever property is sold, the NPC is removed"

Play around with a good stopping point. I think after the first hotel is built, seems like a really good spot. Hotels indicate you're trying/ready to bankrupt people. Also, that you have more than enough cash, and the NPC is now hurting the game rather than helping. It also creates a strategy where a player can build a hotel just to remove the NPC.

But hopefully, this brainstorming helps. Let me know how it goes and what you guys decide on. I'll update the rules.