is generally caused by lack of shocks. Many of the "rice boy cars" i have a few aquaintences with this problem... dorks... The get cheap lowering coils, but leave their stock shocks on, and they are quickly "burnt" out. Or, the shocks they have, just aren't stiff enough in general to keep their stiff springs, causing them to continue bouncing. So, i think that the reason the cars with ipd kits are bouncy, (probably, unless you have also installed hi perf shocks along with...) because the shocks that were equipped weren't up to the task of keeping the springs/car under control. They weren't "dampening" the cars motions... IMO,
Peace,
Kyle