Where could be the problem with my Nissan.

Where could be the problem with my Nissan Mistral?
My name is Aggrey Egessa from Uganda
E-mail: egessaaggrey@yahoo. Com
Tel:+256782963632
I am desperately in need of help. I have a Nissan Mistral model 1994. It gives
me a hard start especially in the morning, but also while I park it for
a long time. When it starts the engine goes off after 2-4 minutes. When
I pump manually the lift pump, the engine remains on. I keep struggling with
this for a long period of time before it eventually starts running normally
. When it starts working normally it goes on for the whole day without
any trouble.
I have so far replaced the lift pump, fuel and oil filters, cleaned the nozzles
, replaced heater elements all in vain. The local mechanics cant identify
the problem.