When i start up my 98 GT it shoots up to about 1600-1700 rpms and sits there for 45-60 seconds without me pressing the gas and then drops down to 600. why is it doing that?
It does it because this is how the calibration is set up from the factory. There are start-up tables and timers that put the RPM up for a short period, probably to warm up the cat converters more quickly and minimize start-up emissions. You can get that initial RPM down if you change a few things in the tune. If you don't want to tune it out yourself (it takes some datalogging and time to get things just right), you can get a custom tune from a reputable tuner that does what you want and, as a bonus, can add a little HP/TQ, too. It only takes money, lol.
I don't know if the later models have the same start-up routine.