hi people im stuck and need help

my xg misses and shakes very bad then suddenly works fine again and when it misses i cant pick up speed

ive changed rotor, cap, leads and plugs and still does it then i thought it might be the fuel pump because it whistles but if i flat stick it a big bang occurs in the exhaust so that must be the fuel not being burnt any ideas

bumping up

check your coil, it could be starting to fail. only misfiring under heavy load yeah?
and that bang your hearing is probably backfire.

Have you checked your timing if you moved the dizzy in any way while you were replacing the dizzy cap etc etc it might be a little out