2002mgzr 1.4 intermitant misfire

I've a 1.4 zr I've changed coil packs, plugs,fuel filter, air filter its been in to the garage so many times its got elastic on there cash register, the fault doesn't show up on the diag machines, I've cleaned the tank out and the fault comes back. Any ideas, also is the fuel tank supposed to form a vacume when the caps on (if not this maybe my problem)

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