2000 Toyota MR2 engine knocking

Hi I sometimes I turn the engine on and it's no problem at all then sometimes im driving and the check engine light comes on and the engine starts knocking about it's like its got wet, then sometimes I go to start and the engine will turn over but will drop right away and turn off.
and by the way while the check engine light is on most the time it comes off within 2 minutes of driving and the car is back to normal but while check enine is on the abs is very tight unless I accelarate thanks it a real headache please reply duncan