Ford Aerostar Stumbles in warm weather

This is an 1993 Aerostar XL with 202000 (true) mileage; auto trans 4.0 L big v6; all power equip. Came with heavy duty trailer pkg. Vehicle has run just fine until last spring & summer when it began to intermitantly display check engine light and stumble & hesitate at both high way speed (60 -70mph) and city speeds (30-45mph) usually for short period (3-5 minutes). Will quit at light if not taken out of gear. AC is broken so not a factor. Check engine light comes & goes. Have not been able to get light to stay on long enough to get to repair facility and read codes. Since this is an OBD1, no memory. Event is totally unpredictable except that higher outside temp is sure to see it happen. Can happen at start or sometimes 3/4 to 1 hr after start. WINTER COLD sees NO PROBLEM whatsover. I am fairly handy with repairs & diagnosis but I don't want to go on a random replacement marathon. I wonder about: coil;plugs;wires;fuel pump; mass air sensor. Plugs have been replaced regularly. Fuel filter has been replaced twice last summer. Gas dryers have been added to fuel frequently. Gas cap seems to fit tightly. Wires are old as is cap & coil (only reason not replaced yet is NO PROBLEM in cold weather). I do understand that heat can cause electrical probs but problem is truly intermitent and unpredictable in duration. There are no current fluid :roll: leaks of any kind (have replaced heads 100k ago & hoses etc are current). Mass air sensor was replaced about 25K ago. I am uncertain what to look at next and would appreciate any help. Temperature is definitely the precipitator and the behaviour is as if fuel starved or ignition electrical breakdown. Any help is appreciated. Thanks.