Engine Warning Light Still Coming On?

I have a T Reg 1999 Corsa 1.2 & it was put on a computer which flagged up 'Lambda Sensor'. This has been replaced, along with a fuel filter & water pump. Still the car makes a whistling noise as you accelerate & the engine light comes on more randomly now, not just when going up hill, which causes the car to 'cough' and lose power. Am now confused as to what I might need to replace next, Ive read the Crankshaft Sensor, Mass Air Flow Unit, clean out the Petrol Pump?? As well as this the back seat (passenger side) is very wet & not drying out, have sat in car & had water poured over it to see if coming in the roof-no-do I need to take the back seat out to look for source. Grateful for any advice you can give please.