Fuel injector problem...

Hey guys. I'm a newbie, so I'll just ask. I have a stock '91 240SX with a stock 174,000 mile KA. Anyway... I just got this car 2 months ago, and now the first and third fuel injectors a slightly "popping up." When I look at the engine, it sticks out a centimeter and causes the engine to shake unevenly when a little throttle is given. When I push the injectors back down the engine runs smoothly, but they eventually pop up again. It's not leaking, as I do not smell gasoline or see any gas anywhere on the engine. Should I replace the injector or o-rings or is it something else? I'm just trying the find a solution for the engine problem until I can learn a lot more with my car, engine stuff, and swap for an SR20. Well, hope you can help me out.