sometimes while i'm driving my brake pedal gets really hard to push down. mostly when i am going fast or have to floor it. its a little scary knowing you can't stop at times. also sometimes the check engine light will flash for a while but go away...any idea what this could be?

 

Sexy, it is sounding to me like you have a wheel speed sensor going. Take your car to a place with a code reader (unless you have one of your own) and see what code(s) you are getting. I am betting you will be getting something along the lines of C11X5 where the X is 4, 5, 6, or 7 (relating to the wheel in question). From there we can give you a better idea of what needs to be done. But, I would be placing money on one of your rear speed rings is rusted and needs replacement.

 

Ide place money on the brake booster pipe check valve deteriorated to the point it allows vacuum to bleed off on accel and loss of brake booster assist. Big vac leaks also creats lean engine codes which will give you the "engine damage result" flashing check engine light. Check and replace the pipe that goes from the top of the intake to the brake booster.Might as well replace the imt orings too with the self sealing green ones too