flat spot in 2.2L AT Telstar

[tx5_telstar]

I have an AT Telstar, 89, 2.2L injected. (And have had 2 AS 2L and have just rebuilt my AT 2L, but haven’t had a chance to put it back together yet, lol)
My 2.2L has recently developed a bit of a flat spot around the 2000 - 3000 rpm mark. It’ll hit a spot, and then it'll kinda die, and when I keep revving it, it picks up again. Hope I explained that ok. I have replaced my leads, plugs, air filter, distributor cap, and the rotor, and still no improvement. I never ever use that crap 91 octane stuff, only 95 or 98 always from a reputable place, usually BP. I have also run an injector cleaner through it. It was supposed to be for 2 tanks, but I put it all in the one tank. It was a red bottle, ngk maybe?
The timing is set at 6 exactly, like the specs say. (But both my other as Telstars I set them at about 14 or so and they ran awesome)
anyway, I think the only thing left is the fuel filter, but this looks expensive. Is it worth replacing it, or is there something else to look at, like the O2 sensor??
Thanks heaps.