Sounds to me like the problem is that either they are on par, or they are almost on par and are benefitting from riding on your coattails (every time a civ researches a tech, the cost of that tech goes down for all civs that civ knows).
There's only so much you can do to pump up your economy on your end; only so many cities can have low corruption, and once those cities have all commerce improvements built, there's not much you can do on your end of the race.
However, you can work from the other end to wreck their economies.
Specifically, instead of trying to horde techs, sell them off for gold; don't wait, that just gives them the opportunity to research the tech themselves and trade it around. Take away their treasuries as fast as you can through trade. Once a civ's treasury is low, start selling for GPT; GPT is where you can really slow an AI. Without a built-up treasury, AIs paying you gpt will have to lower their research sliders to keep up with their payments. Some even overestimate how much they can give you and run out of money, resulting in lots of disbanded improvements throughout their civs. Even if they aren't ruined this drastically, they will eventually become dependant on you for tech; at the same time, they will pump your treasury up to a huge size that alows you to do 4-turn research on any tech you want, creating a vicious cycle for the AI where they speed up your research by paying you gold in order to catch up, then have to speed it up with some more gold to catch up to where that brings you, etc...
Once they become dependent on your research, you can start hording somet echs. You'll still have to sell tech occassionally to keep the gold flowing from them to you, but if enough of their economy is flowing to you, they eventually won't be able to pay for your techs as fast as you research them, and you'll be able to work your way ahead on at least some of the tech tree branches.
