Because it is a a big, heavy piece of metal.
Some 99 Grand Prix vehicles will shift hard when it is warm. Not all of these vehicles will do this.
transmission pressure control solinoid
Is it losing power as you accelerate, or does it do it at idol.
First off, where did you get the 99 grand am strut bar?
accelerate maybe
If it only happens when you accelerate the most likely cause is an axle.
99
Check for a loose catalytic convertere heat shield
The low pressure port on the 99 Grand am is on the passenger side. It is right against the firewall.
Cause it does
two
fast.