i got a 99 teg. it is a LS. recently when i am driving, i saw my temp gauge needle drop down to almost the beginning. then i found out when i put my running and sitting there. the temp will go to normal. but when i drive on the freeway and past 80 mph. the temp drops. and when it drops, i have to pull over 3500 rpm to reach 80. when it is on normal temp. it just need around 3200 rpm to reach 80. what's wrong? anyone knows?