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Since Ford does not sell a Sierra model in the US, I am assuming this is a European or Australian model. As a result, i am afraid I do not ahve any service information available to refer to for exact model specific information on your car. Not that I can't help you resolve the problem, but I cannot tell you exact specific component locations, wire colors, or bolt sizes for example.
The symptoms you describe are a textbook description of an ignition misfire. A lean fuel mixture (such as occurs when the throttle is opened to accelerate) require a much higher firing voltage than a rich fuel mixture does. As a result, if you ahve a marginal ignition component that is just barely able to supply 8KV or so to the spark plugs during cruise or idle operation, it may not be able to supply the 20kv needed to fire the plugs when the throttle isopened or the engine is plaed under a load. Marginal or degraded ignition components usually first exhibit symptoms during acceleration or when climbing hills, although the engine may run normally at other times.
Replacing a fuel filter might be a good idea for maintenance, but it is not likely to be the cause of teh symptoms you described. A restricted fuel filter usually causes the engine to not be able to run at higher speeds (when highest fuel volume is needed), and fuel starvation typically causes the engine to smoothly lose power (not run roughly under load).
You have already replaced the spark plugs; that is a good start assuming they are gapped correctly to specifications and of the correct type and part number for your engine. I would suggest replacing teh other normal parts that are installed as part of a complete ignition tune up service. This would include the spark plug wires, and also the distributor cap and ignition rotor. These are components that need to be replaced every couple of years for normal maintenance anyway.
If finishing the tune-up does not resolve teh issue, the next component to look at would be the ignition coil. An ignition coil that has shorted internal windings can be incapable of generating sufficient voltage to fire teh plugs under acceleration conditions. Start with the plug wires, cap, and rotor though; it is more likely that one or more of those components is causing the problem you are experiencing.
I hope this helps!