Bouncing car, Car bounces when slowing down Options

[gaocs]

I have a really odd thing happing in my car that I hope someone can help with.

About two years ago I had new shocks fitted all round to my 1970 1.7 and since then the car has had an unusual symptom where it "bounces" up and down as you are going along... but it doesn't happen all the time. It always happens when I have been driving at say 50mph+ and then have to slow down, the car then continually bounces up and down as it's going along, especially at slow speeds. This always happens for example when I'm on a motorway and then the traffic slows right down and is then start-stop traffic. It feels like you are on a trampoline.

I always thought it was more of a "seat of the pants" feel, but today my son was driving behind me and he says you can visibly see the car bouncing up and down.

I've had the suspension checked and everything seems ok, so does anyone have any ideas as to what can be causing this? Could it be engine mounts or something else? The wheels are new as are the tyres so it's not a problem related to them not being round. The "bounce" is a few inches of movement up and down.

Thanks.