suspension help

[Gary in BR]

Hello,

After 150k miles I decided to change my oem struts and eilbach springs for a set of coil overs.

I am experiencing "porpoising" basically if I hit a bump or a dip the rear behaves very unsafe. In either case the rear of the car feels like it is leaving the ground, in one instance i know for a fact it left the ground. the other 3 -4 times it feels like it has.

I adjusted the springs to "pre-load" the springs. it helped. The car has only done this 1 time since. I have been told that the spring rate may be to stiff. The coilover company has offered to send me a set of springs of what ever rate I want.

This is where I am stuck. What spring rate should I ask for??

According to the KSports web site the current springs are 6/336 12/671

Any help would be great,

Thanks