1998 Ford Windstar 1998 Ford Windstar P1780; Erratic Trans

Thx in advance for any help! Symptom: Lets say we just started the vehicle for the day ("cold" start 70 Deg F. Weather), drove a few residential streets, then merged onto an expressway for continuous 55+MPH driving for 10 miles. Then exit and continue on residential streets. Come to a complete stop for a red-light intersection. Light turns green and accellerate in normal fashion. The vehicle will start off in 1st gear like everything is normal, but at random times it will not shift up to 2nd nor 3rd gear. The tranny does not slip (eg. The engine has plenty power and accelleration is brisk, but back off on the throttle and you're coasting it's as if in neutral). Now that the symptom is active, slow down to 3 MPH, place the gearshift in 1st gear, and accellerate again. Feels normal, back off the throttle, and the tranny drags the vehicle to a slower speed (which is normal because gearshift is in 1st gear). Anyway, put gearshift in "D" again, accellerate. Back off the throttle after reaching 20-25MPH, coast for 500 feet, then you can feel the clutches "engage". It shifts into the higher gears and you drive like nothing ever happened. Come to a stop again and it's 50-50 chance the symptom will re-occur while at operating temp. The car runs flawless during warm-up period.
A few months previous to this, when coming to a complete stop with gearshift in "D", press on the accellerator and the engine revs with plenty of power, but it feels like you're in Neutral. The only way to get power to the wheels is to shut the engine off, then restart in P or N, and then shift to D. Then drive away as if all is normal. This happened only about 5 times, but may be related to the non-shift-up problem.
I've added an aux trans fluid cooler (in front of the radiator) thinking cooler fluid would benefit. No better. Fluid is still clear-pink, at proper level. Tranny was rebuilt about a year ago; gave 15,000 trouble-free miles. I don't see the correlation between the Ford-Specific P1780 Trans Control Switch being out of Self-Test Range (presumably the OD button at tip of gearshift) and the shifting problem. Other P1780's refer to a Park/Neutral switch, but more so for the Asian market.
Finally, I'm aware that Ford changed the tranny fluid spec during mid-year-production in 1998 from Mercon to Mercon-V. I'm to the understanding that the two are not compatable and cannot determine if the trans shop (a year ago) installed the appropriate rebuild kit due to the mid-year 1998 production change, nor if they even added the correct fluid. Who knows if they mismatched the early-production-year rebuild kit with Mercon-V or vise-versa? The invoice does not say and I didn't ask the questions when the tranny was rebuilt. .I trusted them. Hah.
So, I guess my overall question is: Any advice on how I should proceed? I'm to the understanding that there's a solenoid pack/assembly near the top of the tranny which, if replaced, might resolve the problem.
Sorry for the lengthy text, but I wanted to include all relevant info in hopes for an accurate response. Thx.