Since your main rage generator is the damage you receive crappier gear means more rage :)
So if you are overgeared for an instance you
will have rage issues.
The easiest way to counter this is by changing your gear and using some DPS/PvP gear instead of your regular items. The main point is to lower your mitigation and avoidance (so changing a stamina trinket won't cause any difference in rage gained).
In 5mans this is easy since the def cap is lower so changing one item will probably mean you are still over the cap (535). Another benefit is that your dps will be slightly higher which will result in faster "farming".
When you are running older content raids in WotLK this can be slighty harder to achieve because you should stay above the defense cap (540). But not every tanking item has defense rating (e.g. t10 chest, pillars) + the defense you gain from items will put you over the cap easily (remember, you should not gem/enchant for defense rating if you are over the cap without it), so this can be solved as well (you can also put a def rating enchant to an older - not used - dps/pvp item),
When this is not possible or when progress raiding will mean content "farming" focused rage is really useful, and you can use something like this spec:
http://wotlk.openwow.com/talent#LAM00fZhZMItrx0didIzsGo