What We Say: "It fails randomly"
What We Mean: "I ran this test three times. It failed twice and passed once, and I really don't understand why."
What's the Problem: Random means without known pattern, really. Using the word "random" here is an overgeneralization; you need more attempts to see if there's a pattern or not.
What We Say: "It's slow"
What We Mean: "It ran at 50 MB/s. I was hoping for 100 MB/s."
What's the Problem: Slow means something different to everyone. Sometimes a little slow is okay, but very slow is not. Sometimes slow is what you think, but your expectations are what is in error. Here it pays to be more precise: "about half the speed I usually see in X other test".
Be careful of generalizing too soon. Many of us engineers - developers, QA, support, etc. - thrive on detail. Skipping that detail and going to generalization simply makes you sound either lazy or panicky. So when you're using words that are imprecise:
ask yourself if you can characterize it more specifically. You'll get a lot more help from the people you're working with that way.