The first thing you shoulds do is pull the Throttel body of and give it a good clean. Second new plugs leads and cap and rotor button. You may also find the injecters may need a good clean. I would also guess that it has only ever putted around town all its life, the worst thing you can do is flog the guts out of it until you sort out all of you problems. You may also whant to check the TPI switch setting.
I had a very similar problem on my EFXR8. My solution was that the MAF sensor unit was faulty, the owner before me had a finer filter and had over-oiled the filter, so the air flow meter was fowled. I cleaned it as best i could but the problem persisted.
I ended up borowing a mates maf sensor and it ran well with his sensor on. I ended up doing a bit of research and bought a GMS 75mm MAF sensor and i havent had a problem since. My sensor also tricks the computer so that it runs 15 percent leaner, so it takes a little bit of the richness out of the system.
I had similar symptions. It was quicker at half throttle than it was at full throttle. Very anoying. It has been going fine for the last 50,000km, but i clean the maf every 10,000km.
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