'92 F150 - thermostat problem?

My '92 F150 (168,000 miles) has been partially retired for about a year. I use it to take the garbage down the driveway, and haul things here and there. About 2000 miles a year.
I just noticed that the heater was blowing lukewarm air. The temperature gauge was on the high end of normal. Then, the gauge went down to the low end of normal rather rapidly, and the heater was blowing regular, warm air. It then went back up and blew the lukewarm air. This cycle repeated about every few minutes.
I fully suspect this is a bad thermostat, correct?
Todd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Or the coolant is so low that there's mostly air in the rad and heater core, and once the thermostat opens, the rush of coolant leaves the temp sensor exposed to the repositioned air pocket (giving a false low reading). The air pocket could also affect the thermostat, letting it close again while the coolant-deprived engine block slightly overheats in spots, and the resulting steam forces some coolant up into the heater. Eventually the coolant makes its way back to the block, covering the temp sensor, draining the heater core, and the whole process repeats itself over and over and over and.....
Or this could all be merely the ramblings of a madman.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.