1997 Chevy Cavalier In Danger

I have a 1997 Chevy Cavalier that acts up mainly first thing in the mornings or when im going to park in a parking lot somewhere, when car is going at a slow speed. The car doesnt not brake readily, it takes about a few seconds for them to catch it seems and when they do the car shutters and there is sometimes a buck or kick from the rear of the car. This only happens intermittently, when i take it to the mechanic the problem does not occur. Also, there are new front breaks and rotors and new tires on the car. I was told a couples months ago that my rear brakes were good. BUT then what could be causing this?? This has been going on for about 6 months, and getting worse. I thought when the front brakes and rotors were put on that, that would fix the problem, it didnt. I frear to drive this car in snow or ice because i never know when the "no breaks for a few seconds" is going to occur. It has been running loud lately also, i don't know if thats another problem or part of this. thanks Estella