ford fiesta 2001 zetec 1.25 problems HELP !!!!?

Hi all, Have a nightware with my 2001 ford fiesta 1.25 zetec se ghia 5 door i bought the car as a non-runneras the previous owner over filled it with oil and he hadn't servided it in 2years either so i saved the poor motor haha :) now ive done a full service - spark plugs, oil, oil filter, pollen filter, air filter, full coolant change. Car was running great then i found the Brake master cyclinder was leaking brake fluid so i fixed that then i start the car again and everything when bad now the car is mis-firing, choking, shaking, water like liquid coming out the exhaust and the revs are bouncing even when idle in neutral and i can not control the revs onlys at 4,000 and the engine management light is flashing on and off randomly and i mean RANDOMLY like sometimes when i start up the car or sometimes when i have it running idle for a while. Now i'm a 3rd year apprentice mechanic so i buy and sell cars to help my low income. Now i've checked the coil and plugs there all sparking the leads are carring current fine theres no rips or corrosion on the leads or coil pack. When i first got the car it did have the engine management light on and the code was PO420 i cant remeber it was catalytic convertor treshold something anyway i didn't have alot of money so i went against my gut and bought a bottle of cataclean it was meant to fix the problem with the catalytic convertor and turn of the engine light but to my surprize it worked 100% amd done everything it said it would do ! anyway any advice or experience in what my problem may be ? i'm thinking trottle body/ mass air flow meter or EGR Valve ? thanks