YOU WANT YOUR TRANNY TO STAY IN LOW GEAR. there is a certain amount of throttle that will tell your transmission to shift into a lower gear. usually this is at or near wide open throttle (full throttle). The car was designed to do this and if the tranny does this with out any thumping or clunking then it is perfectly healthy. the lower gear raises your rpms and puts your enine spped in a prime torque area on your tachometer. this is what you want. the reasons why automatic transmissions are designed to do this is for safety. this gear drop is intended to be used for passing and that extra power you recieve when you step on it and it drops into a lower gear is often reffered to as passing power. this allows drivers to pass cars faster which is safer. If you drove a manual transmission vehicle you would understand why your transmission does this. manual drivers do this gear drop on their own during spirited driving and its completely normal. i know because i drive an Audi s4 with a seven speed manual transmission and i just thought i would clear some things up for you guys . so yeah, its what you want to happen and it defianately isnt your tranny slipping out of gear.
huudoo you may want to check your transmission fluid level. if the level seems to be fine then check the fluid itself and see whether or not it needs to be changed. i'd suggest changing it yourself and using a higher quality transmission fluid. this should solve your problem.

ha im only 17

happy driving guys.