Suspension issue: Springs or shocks/struts?

Hi guys, got a weird problem..hope someone can explain if its normal or one or both of these that are the culprit.

I bought this 05 tdi with 150K on it...the last owner said he put in aftermarket suspension cup kit about 20K ago. it does feel good..but when i go over a speed bump or into or out of a driveway, there tends to be a drop in tension. almost like the suspension bottoms out, but it actually doesnt...it goes down pretty low on its initial bounce but is easily recovered without a second bounce. so are the springs to weak or is it the struts/shocks?
I am with winter tires right now, so could that be the problem?

TIA.

I don't see how one could offer proper advice from afar.

I'd seek the advice of a local VW guru (not a VW dealer) who understands A4-chassis suspension geometry, have them inspect, test drive it, and make a recommendation.