user7500: Tournament players can "have a good tournament" because of the psychology involved, so their playing strength will vary from game to game. Do computers play at a constant strength all the time, or do they have the occasional bad tournament?

user825: We're going to assume there is no hardware issue, as pointed by Tonny Ennis in his comment.
It's possible that a computer engine experiences a "bad" tournament, but it's extremely rare. When we say "bad", we don't mean collapsing like blundering pieces, it's more like not getting a good position suitable for the engine. Computer engine always play at a constant strength, dictated by the algorithm. They won't do anything other than what the algorithm tells it to do.
Let's give an example, it's well known Komodo is a better engine than Stockfish in a strategic position. If we randomly choose the openings in a match of four. We might see a closed position in three of those games. As expected, Komodo outplayed Stockfish in those three games. The final score would be 3-1 in favour of Komodo.
Does that mean Komodo is a much stronger engine than Stockfish? Probably not. Can we conclude Stockfish had a "bad day"? Maybe. But please remember the sample size is too small, what if we let them play more games? A typical computer chess tournament can have a match with over 1000 games. If we do it, we might see a very different result.
Conclusion: if we see a chess engine has a "bad day", usually it's because the sample size is too small or the testing condition is biased. The law of large number will erase unsystematic variation given enough sample size. 

7500: <p>Tournament players can "have a good tournament" because of the psychology involved, so their playing strength will vary from game to game. Do computers play at a constant strength all the time, or do they have the occasional bad tournament?</p>
 825: <p>We're going to assume there is no hardware issue, as pointed by Tonny Ennis in his comment.</p>

<p>It's possible that a computer engine experiences a "bad" tournament, but it's extremely rare. When we say "bad", we don't mean collapsing like blundering pieces, it's more like not getting a good position suitable for the engine. Computer engine always play at a constant strength, dictated by the algorithm. They won't do anything other than what the algorithm tells it to do.</p>

<p>Let's give an example, it's well known Komodo is a better engine than Stockfish in a strategic position. If we randomly choose the openings in a match of four. We might see a closed position in three of those games. As expected, Komodo outplayed Stockfish in those three games. The final score would be 3-1 in favour of Komodo.</p>

<p>Does that mean Komodo is a much stronger engine than Stockfish? Probably not. Can we conclude Stockfish had a "bad day"? Maybe. But please remember the sample size is <strong>too small</strong>, what if we let them play more games? A typical computer chess tournament can have a match with over 1000 games. If we do it, we might see a very different result.</p>

<p>Conclusion: if we see a chess engine has a "bad day", usually it's because the sample size is <strong>too small</strong> or the testing condition is <strong>biased</strong>. The law of large number will erase unsystematic variation given enough sample size. </p>
