overheating...again

in the past ive had some overheating problems, i replaced the tstat with a 180* one, got myself a 3 core radiator, and thought the problem would be solved. i dont have a blown head gasket. it runs perfectly while im driving in my higher gears like on a freeway but when i hit the low gears in the streets the temp creeps up. what could this be? please help me out. i let the car idle on saturday for a good 40+ mins and it never overheated, i took it for a spin around the block and it seemed ok. today i drove it across town what should be 30 min drive, and this is when my problems surfaced with the creeping temp. as soon as the temp was rising i pulled over to take a look and i hadnt fully tightened the upper rad. hose was this my problem?

hissin, factory gauge, fan and fan clutch im not too sure on, seem ok. im assuming that when the upper hose wasnt tightened down, maybe some air seeped in?