1998 Dodge Caravan Starting Problem

The van is recently having a problem with the starter not engaging. The solenoid clicks but the starter does not engage. The starter sometimes engages if you keep engaging the ignition switch.
I checked the battery voltage and it was low. The terminals were corroded so I cleaned the corrosion off with a baking soda and removed the terminals and cleaned them with a wire brush and reconnected them. I charged the battery so there is 12.8 volts with the ignition off. The starter worked and did not have a problem for three days. Then the van would not start with the solenoid clicking. The voltage is now at 11.8 volts. I charged the battery again and with the voltage the starter did not engage but the solenoid is clicking. When it does start the voltage at the battery rises to a maximum of 14.2 volts. I am of the mind that something was causing the battery voltage to drop while the vehicle is off. I had the battery checked it checks good with the rated cranking amperage.
Does this appear to be more than one problem? Can the solenoid be replaced separately on this starter and does the voltage regulator appear to be working correctly.