Just so I understand this - you finished TBR practice tests with overall less time (or at least less timing woes) than AAMC tests? That to me is suspicious because TBR is notorious for being more difficult than the AAMC - then again you didn't disclose your scores.
Despite that difference, what it looks like from my shoes is test anxiety. Are you becoming too much of a perfectionist on test day? Are you trying to prove every last bit of an equation to make sure you have the right answer instead of taking that last short cut (because you have and can do so in the past)? This may be what's happening. You may be telling yourself on test day that this is it, that it was all for the next 5 hours, and that may be too much hype. That may be affecting how you perform. Calm down, go in with confidence, tell yourself your ready, and attack it like you do practice tests (though spend the remaining time working on becoming just a touch quicker).