I had that very same problem and just fixed it today. After waiting patiently until 101-100 BCE for the Marian Reforms, I ran into a persistent CTD whenever they were about to kick in. Like you, I met all the requirements along with a Marius wanabe. Even when I scripted the reforms I still got a CTD when they were about to kick in after the Eleutheroi turn. I did not have this problem in previous releases.
After forty-eight hours of experimentation I found the culprit. Early in my campaign I noticed the Carthage Reform spam was taking place. I downloaded the fix and my game continued undisturbed for another hundred years, so I figured it was case closed. The cities I took from Carthage were full of reform markers and looked ugly but everything worked fine...until time for the Marians. I isolated the problem by gifting those cities and then the reforms activated properly with no CTD. As further proof, when I scripted the reforms to take place in a saved game I had that pre-dated the Carthage reform marker spam, the Marians kicked in properly.
Obviously I wanted to keep the Res Publica growing historically so gifting is not a solution.
I opened the export_descr_buildings file, looked up Reform 2 (the one that affects Carthage), changed "building hinterland_reforms2" to "building_reforms2", reloaded my game and my cities were clear and the Marians triggered properly with no CTD.
Have no fear, the Carthage reform marker reappears after a turn as well as the other faction's markers affected by reforms2 (and if you have the fix it does not spam again). Afterwards you can change the building_desc back to hinterland. Problem solved and I have had no problems since.
Now my professional legionnaires are on the warpath.
Hope this helps.
ROMA INVICTA