New boxter, last owner chopped springs- cause fishtail or tail wag at high speeds??

[Kboggs]

Just got my first boxster yesterday. 99 2.5 tip with 87k.

It has been lowered, ride was not firm as with most that are. Coming home at 50-60 mph plus the rear end feels at times like it is skating or on ice is the best way to describe. I already found a used set of oem springs to put back on it as i know that chopping springs is no good? Me and a nearby shop looker underneath and all control arms, bushings, and everything visible looks good? Does this symptom sound like something caused by chopping springs? It has to be lowered close to 2 inches, maybe more? Rides soft with no noises???? Tires are brand new also.