2004 Hyundai Accent Gear shift hesitation

Sometimes when pulling out from a stop - especially when going uphill - The transmission seems like it has trouble switching into the correct gear, resulting in high rpm with little acceleration. Once it finally kicks into the correct gear, the car sorta jolts foward, sometime making an audible "crunching" (?) Noise. My last car had this problem right after I had the transmission fluid flushed, and the transmission died shortly after.

So, my questions are, how does this happen, what can I do to fix it, and how can I prevent it from happening in the future?
Please help, as I have 1.5 years left to pay this car off, and can't afford a second car payment or a new transmission!