The way you are describing the 'bounce' and 'moving up and down' makes me wonder if the shockers absorbers are shot.

You haven't described it as rock hard with no movement, which would have me suspecting aftermarket hard or lowering springs having been fitted at some point, and/or standard bushes replaced with nylon sporty things in garish purple.

Presumably you've had a poke nose underneath and inspected the springs etc.

Have you tried a bounce test on it? sometimes it can be illuminating following your own car whilst someone else drives it along undulating roads, with a bit of road positioning you can see how the suspension is reacting all the way round the car.

Tyre spec sounds fine to me unless someone's stuck 50psi in each.

I always found Vectra seats horrid things, for too hard for my delicate little behind.

>> I was told some of the springs had been replaced

I suspect that's your problem. Could the rear springs have been replaced with something stiffer? Has it a tow bar, or evidence of it once having one? Spring assisters?

If it's been lowered the springs will probably be stiffer. Could you find what the ride height should be and measure yours, or compare it with another?