Your truck is in the mid-year range of TFI ignition module Fords. The modules are prone to heat failure at some point and the problem often surfaces in the summertime. Symptoms vary but can very well match what is going on with the truck.
The module is the little gray rectangular widget on the side of the distributor and testing them is usually a waste of time as short of a complete, total failure they usually test good.
Do a net search for TFI Settlement and you can read all about those modules. It will be too late to get any reimbursement for repair costs but the info will explain the whys behind the module problems.
If you change the module you MUST use the special grease that comes with the new module and you will also need a 4MM or 7/32 deep socket or nut driver to access the 2 screws. Hope that helps.