Sounds like how my Clio with the E7J engine used to behave. I tried all the usual cheap(ish) suspects: air and fuel filter, new distributor, rotor and HT leads and spark plugs (always had healthy looking colouring). Took exhaust off and checked cat (its honeycombs were clear). Did compression checks (fine), and Redexed it to hell. Did resistance check on coil (fine). Checked valve clearances about ten times. I even took ECU apart and checked capacitors and did any dry solder joints. My brother's old Clio 1.2 still outperformed it massively, and it struggled to reach 70 on the motorway just like yours.
Never did get to the bottom of it, but I can tell you what I didn't change, which may narrow things down: HT coil, lambda sensor, map sensor and injector. I was told Lambda isn't used for short while after engine started (is that true?) and told to disconnect its wires, start car and have quick drive in it to see if it's better - in my case it was no better at all.
I don't know if it was related to the fault, but mine used to surge when slowing down just before coming to a standstill, made me quick on the brakes to avoid a slight knock to the car in front at the traffic lights! I'll be interested to see if you get to the bottom of this, as I never did, nor did I find the cause of the incredibly stiff steering that made my mum refuse to drive it. Now I'm on to new cars with new problems
