Polo MK3 not starting all the time?

hey everyone,

i've just got my hands on a mk3 polo, fitted a new engine in it, it was originally a 1.0 litre, but it was then changed to a 1.3 (MK1 golf) engine, unfortunately that's blown, so i went and bought a nice little 1.0 again.

fitted it....

it starts on the bump....engine sounds so nice and quiet.

switch it off, and it sometimes starts then it doesn't?

any tips?

many thanks

Ben