As a PLD main, during dungeons I can cycle my buffs for covering nearly 100% of the time spent taking damage while still reserving big buffs for big damage (this is at about level 50). My hp rarely dips and dives in dungeons except with healers (like myself) who try to pump every bit of DPS when they can, but I also prioritize survivability as a tank and only add in DPS accessories if/when appropriate, whereas often times I'll find I'm healing tanks in full DPS accessories their first time through a new instance at minimum level/iLevel. The ladder are the tanks that slow down runs and occasionally cause wipes (or with inexperienced healers, can cause failures).

Another thing I'm seeing right now is an issue with DRKs--not the job itself, but the fact that it's a fun/attractive job means that there's a lot of people playing it who have never tanked before or may not even have interest in learning the role. At your level, the biggest noticeable difference between a good and bad PLD is their ability to manage aggro, but for DRK the difference is squishiness.

Lastly there's WARs, who have a deep hp pool for lower mitigation, but also self heals. So it's not unusual for their hp to fluctuate a bit more. Though at higher levels they can maintain their high hp with very little dipping and diving when played right (in your average dungeon).