Starcraft

So my laptop is an inspiron e1505 and it runs pretty good. World of Warcraft runs perfectly fine, but when i try to play starcraft, it doesn't work. i can enter the game and connect to battle.net. however, when i try to join a game, i get really really stinking laggy. i chatted with a dell representative and upgrading my directx and something else worked.....for a few hours and now it doesnt work again. i'm thinking about just sending my laptop to a dell store and just getting a new one or something since i am still under warrenty. but i would just like to fix starcraft. please help, this has been going on for a long time and i didnt buy this laptop to be ripped off of playing games or using other software.