Macbook Pro 13' core i5 freezing constantly, what should i do to fix this?

i bought my mac last week and to my surprise it started freezing and lag , having the spinning beach ball all the time , i spent hours reading on these topics but found no solution. Later on i sent it to the local apple technical service to ensure if theres any problem , had the hardware test done , lately we found no problems. they said they cant report to apple until they see what im having. And as soon as i brought it back home , it started freezing. especially on bootup and wake-ups. why that ?

Well apps that i use are : itunes , facebook on safari , skype , imessage thats all.

my mac has only 5 gbs of songs thats all with 475 gb free on hardisk

Macbook pro 13 core i5 2.5 with mountain lion 10.8.3 on it.

can somebody help me sort this out ? i trusted apple but in this case , apple let me down .