You are correct, 13.8 to 14.2 volts is considered charging voltage. But keep in mind that at idle, sometimes the alternator doesn't put out as good as it should. Also if there is a bad connection between the terminal block behind the battery and horn relay you can have a similar issue. The regulator will keep the system voltage at spec but a charging battery will be less than the regulated voltage. If this is the case, cleaning the connection, replacing the terminals etc. will get it back. The regulator may also be wimping out on you. Are you using a mechanical regulator?
Once you determine the charging system is up to par you may find you are fighting a heat soak problem. Are you running headers? There are a couple of theories behind heat soaking. One is that the solenoid doesn't fully engage to get the starter to kick in. Another is that the heat causes the winding resistance to increase in both the solenoid and the starter windings making it crank poorly when hot. A heat shield is the usual fix for this. There are also guys who use a ford style solenoid to isolate it from the heat source. But before you get into any of that, make sure the charging system is up to par.
