Toshiba??????????

so i have a question about my computer. i dont know why but everytime i go on it it shuts down after every 30 min. it is geting on my last and only nerve! what can i do to stop this, i do have a "service plane" for it but i have had this computer for 4 years or 3 i dont really remember when i got it. is so weird because they were suing the Toshiba company and i did not join because then cause my computer was doing good but all last month and this it has gone bad. and yes i clean it i have window washer and all the other crap.so can i still get it fixed or do i have to pay mor and also this computer was coast alot so its no cheep ****.