missig bad after 20 miles or so a small miss otherwise only code evap leak but has been there before miss started
after 20 to 30 miles my truck picks up a bad miss at lowend and overdrivw on 1999 Ford F-150
From the description it sounds like you have a type-a ignition misfire, the threshold for setting dtcs on older fords is broader so they won't set a code as fast as a newer ford. You'll need to take it to a shop that has a power balance test capability that will show exactly which cylinder is misfiring to get an accurate diag.
poss alot of things do you have any eng codes?
Also you can have a cylinder contribution test performed with a scanner, most likely an ignition coil or coils failing.