[The following is based on my experiences in 3.5e, but from what I know about Pathfinder it should be trivially adapted. Also, I apologize in advance for what I'm certain will be a post filled with incorrect terminology -- I've been playing 4e for quite some time now, and it's been even longer since I last sat down with 3.5e.]
If you think Diplomacy is broken at level 5, just wait until you get to the Epic levels! This is where we were when my DM decided to address it.
They way he approached it was to completely ditch the static DC list -- static DCs make sense for climbing ladders (which don't typically get harder as you get higher in level), but they don't make sense at all when you're dealing with more and more experienced and powerful individuals; just like AC and other such things, as the CR goes up so, too, must the DC.
So he sat down and took the table of Diplomacy skill DCs and turned them into situational modifiers. I think he started with "Neutral" granting a +4 (reasoning being that changing people's minds is not easy, even if they don't dislike/distrust you), and then each step toward Hostile added an additional +2, while each step toward Friendly added a -2.
The resulting modifier was then used on the NPC's own opposed Diplomacy roll. Thus the table of Diplomacy DCs that is so trivial for PCs to game was gone, replaced by opposed checks to modify a character's attitude.
But he went even further. Between each stage on the "trust continuum" (i.e. Friendly, Neutral, Hostile, etc.), the DM added a "half step"; a successful Diplomacy check would move the NPC's attitude half a step, not a full step, thus requiring 2 successes to effect a change in the character's attitude. (When an NPC is on one of these "half steps", his/her attitude is the one "rounded" toward neutral; thus an NPC is effectively Neutral across 3 distinct "steps", but 2 "steps" for all others.)
Finally, he added one more thing: Continued successful/failed checks could move an NPC further than the ends of the "attitude spectrum", although no further mechanical advantages were earned. What it did do was make it less likely for the NPC's attitude to be changed later, by simply keeping track of how many "steps" would need to be adjusted.
These were the mechanical changes he house-ruled into Diplomacy. He also required certain role-play elements to also be met before a Diplomacy check could even be attempted -- the Halfling Bard walking up to the dragon and rolling an impressive 34 Diplomacy is just wasted effort if said dragon isn't even listening! There were also common-sense limitations imposed: a dragon who's entire life is centered around accumulating his horde is not going to just give it up, no matter how many Diplomacy successes the Bard accumulates!
Best Answer
If your game allows characters to be created with traits, Pathfinder includes two social traits that do this, and there's a few other similar effects.
Student of Philosophy, social trait from PPC: Quests and Campaigns
This doesn't apply to absolutely all uses of Diplomacy, but it does extend the benefit to some uses of Bluff as well.
Clever Wordplay, social trait from PPC: Pathfinder Society Primer
This allows you to substitute Intelligence for Charisma entirely on any one Charisma-based skill, such as Diplomacy.
The Empiricist, investigator archetype from PRG: Advanced Class Guide
The Empiricist's Ceaseless Observation class feature allows that:
Which broadly allows you to substitute Intelligence on several skills, and specifically on Diplomacy checks to gather information but not other Diplomacy checks.
Ironclad Logic, feat from PRG: Ultimate Intrigue
This doesn't actually let you substitute Intelligence for Charisma generally, but it does represent leveraging your high Intelligence (Int 19 required) for a bonus to Diplomacy checks:
Inside Man, slayer talent from Advanced Options: Slayer Talents & Lethalities (third-party, Rogue Genius Games)
Besides a bunch of free class skills, the Slayer gets to substitute Intelligence on Diplomacy checks: