Check all the main battery leads are nice and clean, good contact, also try the wire trick, use a bit of wire direct from the battery feed at the solenoid, to the solenoid feed from the ignition switch, so as to bypass the ignition switch, see if that does anything, some suffer from a high resistance on the solenoid terminal(in the loom), often resoldering these solves the problem, but remember to use some adhesive lined heatshrink to seal the join from moisture and breaking.
tabetha