what would be casuing really crappy throttle response? It also seems to be an intermittent problem, so one day it feels perky and powerful and then the next time i start it it will be sluggish and powerless.

I have tried only running on 98 octane fuel which seems to help but i still get poor response sometimes. I have had the fuel filter changed recently which pretty much got rid of the problem for a couple of weeks but its coming back.

The car is due for a service and i change the plugs every 5000kms so they are not really that old, but could they have something to do with it?

I have figured out that fuel from a couple of servos in my area makes the problem worse so i avoid them but the problem still gets progressively worse.
its a 97 EL ghia with 235,000kms

plugs should last for at least 20'000km..

yeh thats what i thought. I just change em coz they are cheap. should i be looking at other ignition components though? leads etc? I have heard that cutting a couple of holes in the airbox can improve response, but that wouldnt be fixing the problem..

u might have a dirty fuel tank since when u changed ur filter it went away but has came back

as gusbus said. If you can be a***, clean the fuel tank and replace the filter again. See if that helps out. Also it might be a dying fuel pump too if it hasn't been changed before.

Also stuck/old/stretched throttle cables can also contribute...remove it and lube it, see if that helps.

Spark plugs every 5000km and cheap spark plugs? I mean they are relatively, but if you drive over 500km a week over a year, that's a lot of spark plugs.

also look at your ignition coils. Thats what has gone bad on mine a I had really crappy throttle response. I had to replace all 6 of them this past monday as one wasnt working and 2 had low ability to help produce spark for the plugs. check ingnition wires aswell. You can also try to get octane booster with 98 ron for two tanks and see if it progressively gets better, cause if you think it was dirty fuel then your injectors could clouged, I use octane booster every 1 to 2 months just to make sure of cleanliness! But do both have someone check the coils and try octane booster.