Is there a mod that means you need to gain more EP before you can level up?

There are a bunch. Using a variety of different methods. Search nexus for "leveling" or "level".

That's not really what you want. It was tried, but resulted in compatibility issues (such as a major one with Broken Steel).

A better way of doing this is to reduce your XP for every thing that you gain XP for (while still requiring the same amount of XPR to level up).

That is how my http://www.fallout3nexus.com/downloads/file.php?id=7674 works, and it comes in two variations (a normal Reduced XPR, and an Extreme version).
And, if you want to level up even slower, you can also use my Reduced Quest XPR module (which comes with my http://www.fallout3nexus.com/downloads/file.php?id=7565, which reduces all the quests experience points you receive by 90%. Which means that you will no longer level up when leaving the vault, but will be out in the Wasteland, trying to survive at Level 1.

I am sorry to take this thread a little off topic but if I may ask, why does it cause compatibility issues? It doesn't seem like Broken Steel modifies the game setting iXPBumpBase which is the main factor in how much experience is required to level up if I am not mistaken.

I only ask because I was planning on having the next version of Experience Control allow the Player to modify the amount of experience necessary to level up.

i have been using one of those slower levelling mods for months now.. I have NEVER had a problem levelling with broken steel installed. In fact, the only problem I ever had with levelling was completely unrelated to slower levelling mod, the problem happened to be with FOOK which wss fixed with one of the hotfixes.

Hey, I'm just trying to warn the original poster here. Many people who have tried a slower leveling mod that changes the total required experience points needed (changing iXPBumpBase) for each level have indeed had problems when Broken Steel is applied (as in no longer being able to level up).

There have been two basic approaches to slowing down how fast you level up.
1.) increase the required number of total XPR needed to level up, which involves changes to the iXPBumpBase (which CAN cause problems).
2.) decrease the number of XPR you receive for everything that you gain XPRs for.

#2 is generally considered to be the preferred method. I really don't care if you want to use my mod or not, I just offered it as a possible solution.