Intermittent flattening

[Mags57]

I love my car, but there are times when I just don't understand it! It's a Laguna II dCi 1.9/July 2001 sports tourer [dymanique - if that helps anyone?] and there were lots of days when it was clean out the stable door and no catching it, brilliant. Now it'll falter between 2 and 3,000 revs, flatten completely and then pick up again. We can end up kangaroo-hopping until it breaks the 3,500 barrier then we seem okay!! My lovely Renault guys reckons there's nothing they can do till it shows a warning light, the diagnostics are clear. They've told me to replace all four glow plugs, put in premium grade diesel every now and then, top up with STP and change the oil every 6,000mls. I do this anyway, other than the glow plugs which I'm told only start the car and have nothing to do after that!! We've been kangaroo-hopping for at least 6 months now; and I've had enough. Surely something is wrong with something...? What, where; and how much damage is being caused? I don't pretend to know anything about engines, I don't. I like cars to, well, just go! It's done around 75,000mls, new belt etc. Please, can anyone help me?