bay

The car is a 95 wrx import with mods (cat back exhaust panel air filter and a dvalve)It has got a fault that just comes and goes. When ticking over it sometimes revs 2000 revs then drops down to below 1000 revs and the engine check light will come on but if you rev it to 2000 then it goes off and doest come back on so i have plugged the leads together to see what comes up and it has come up with two. THE first one come up with 4 long and 5 short flashes i think that is 45 and the 2nd one is 2 long and 4 short i think that is 24. Does anyone have any idea what it could be thanks.