An '02 Ranger with a distributor? They only come with 3 engine options that year: 2.3l, 3.0l, and 4.0l, all of which are distributorless ignition. Are you referring to the coil pack?
It could be a number of issues, of course any comment on here is pure speculation, but the way you describe it does seem to be ignition related.
If it is a 4.0l engine, they very commonly have a problem with heads cracking. Quite often its not bad enough to cause a noticeable coolant loss (although it may), but what it often does is causes minor deposits on the sparkplugs which cause them to misfire. If replacing the plugs cures the problem for a few days, such is likely the problem.
If you have access to a scan tool, Fords are quite good at setting codes for misfires, which may lead you to the problem cylinder(s). Cylinder misfire codes are In the P03XX range. P0301-cyl#1 misfire, P0302-cyl#2 misfire, etc... with P0300-meaning multiple/random cylinder misfire.
Of course this website is all about messing with stuff yourself, but you could save yourself a lot of time and money by getting it professionally diagnosed. A good resource for finding a qualified professional in your area is to check on http://www.iatn.net/
There are many on here that will continue to give suggestions though if you decide to keep slugging away on your own...