Transmission codes 713 & 734

[jzbeaver]

The O/D light on my 1998 Windstar 3.8 recently started flashing, usually after driving at 65-70 mph for 20 miles or so. I had the fluid changed (it had not been changed for a while), following recommendations here, and at first I thought that solved the problem, but it has returned. Operationally,it seems like, when the light is flashing, that there is some slippage, as if the tranny is alternating between 3rd and 4th, intermittently-- esp. on inclines. But most of the time,
it seems normal, even with the light flashing for a long time.

I have taken it to 2 tranny shops, recommended by a reliable repair shop. The first one claimed that the 713-- temperature sensor-- was not significant, as it is the 734 that is causing the hight temperature. He cleared the codes, and said that if the flashing recurs right away, it is a harness problem, but if it takes a while to recur, I need a complete rebuild.

On the way to the second shop-- about 10 miles-- the O/D light began to flash again (driving 55-60)- much sooner than usual, but not right away.

At the shop, the 713 appeared, but not the 734, no doubt because the code had just been cleared. This shop says he can't determine the cause, but thinks it is the torque converter lock-up malfunctioning-- which seems consistent with the 734 code. He says I need to leave the car with him for 2 hours so it can be driven to see how it shifts. Further, if it is a lock-up problem, the tranny has to be taken down to fix it-- expensive, but this is not a rebuild. He also says that the fluid did not smell good, but that was not likely a cause, but a symptom.

I don't know whether to believe either of these explanations. Would it make sense to have the TRS and/or some of the shift solenoids replaced, which are involved in these codes-- even though no code has appeared indicating their failure?
Also, since the fluid change was not complete-- only 7 of 12+ qts. because fluid in the converter was not changed-- would it make any sense to get a complete flush before doing anything else?

Appreciate any advice on this.