Fuel starvation + electrical weirdness

[moomin]

On a 1984 Coupe GT5E (JS Engine)

Recently I replaced my fuel pump and relay as the relay had died, has been fine for three weeks until yesterday it developed a fuel starvation misfire.

80% of the time
Start car
Relays click
Fuel pump turns on
Car starts
Drive off it starts to misfire
Then clears up
Then comes back
repeat until stopped

10% of the time
Fuel pump won't fire at all
Bash the pump or bash the relays makes no difference
then randomly it'll work

rest of the time its perfect

Problem can't be forced to happen under; load, accerlation, deaccerlation, heavy cornering, driving over bumps or any other abuse.

Seems to be random

HELP!