Have a specific plan– something better than "buy some coins and wait til I'm rich."

You should be able to answer these two questions above all else:
1. How much am I comfortable losing?  What's your plan if (actually, when) coins tank? Cut your losses?  Or buy more?
2. What's my exit plan?  Cash out when you've made back your investment and reinvest the rest?  If you never sell, you never make money.  You can hodl BTC forever- or you can cash out some gains at predetermined price levels.  Pick a risk/reward balance you can realistically follow.

Go ahead and write them down.  Put it on a sticky note on your desk.  This way when you wake up to see your holdings are down 30% you don't panic sell.  You don't even think.  You have your plan in front of you.

#1: Does the use case make sense? Who does this coin solve a problem for? (besides the people selling it!)
#2: Have they made any concrete progress on their roadmap?  Is there code on github?
#3: Is it closed source?  If so, the odds of it being a scam are much higher unless they have a really good reason.

Crypto's a new frontier, but supply and demand determine price- same as with anything else.

Of course, demand for crypto's super volatile and often irrational.

Price and market cap don't tell the whole story.  Looking at depth charts (how much supply and demand exist across different prices) on exchanges is pretty illuminating.  For instance, a low key altcoin with a market cap (price/coin X number of coins) of $1,000,000 might see its value drop 25% after one big holder tries dumping just $10,000 because not many people want to buy.

As for what causes these swings... algo trading, pump and dump manipulation, consolidated ownership among a couple of whales, mass hysteria... barring some major news story it's usually hard to say.

If it's between just two 1080ti's 5-10% could be within the natural variance between particularly good/bad chips.

Another factor could be as simple as temperature.  Depending where you put the rigs one might have better cooling (i.e. on a cold window sill vs in a closet.)