ignition problems

Recently ive been having ignition problems. after the engine has run for a while, the spark gets weak. i have replaced the entire ignition recently trying to buy my way out of this problem, to no avail. acts like the coil is getting too hot, so i put a voltage meter on the positive of the coil running to see if it has the correct voltage(6-9v). the meter goes crazy never giving a reading! maybe the ignition switch??