1995 Toyota Avalon harsh trans shift problem

I have 1995 Toyota Avalon w/173K miles. It’s in good shape mechanically and cosmetically. It was my father-in-laws car who purchased it new. He’s a cheapskate and couldn’t bring himself to give it to his granddaughter, I had to buy it from him for a princely sum, well not so princely, but more than I wanted to pay. My 16 yr old daughter is the primary driver.
I have extensive service records and have replaced the springs/struts, o2 sensors, and a few other bits.
My question is related to hard shifts and a check engine light. During the summer the trans shifts normally. In the fall & winter (Chicagoland), when temps dip below 50 degrees, the trans shifts harshly and the check engine light blinks indicating a trans solenoid is defective. The trans fluid has been flushed twice, most recently in Oct 2008 at the Toyota dealership. My theory is the techs filled the trans with a heavier weight fluid than is necessary and that is triggering the codes.

Thoughts anyone?

Are you saying that the codes have actually been read, or are you assuming that the flashing light is something about the transmission? You need to post the actual error code(s). Typically the check engine light will flash due to a misfiring engine. I suppose a '95 Avalon could be different, but clarify exactly what you know about the error codes and how.

If a transmission expert (not just a general mechanic) hasn’t looked at it, that is probably good idea.

While incorrect fluid is often a reason that people have issues after transmission servicing, if you do have a trans solenoid-related code then it can very possibly have nothing to do with the fluid. Many of those codes are about electrical faults and given your description I’d suspect an actual defective solenoid. After the car gets fully warmed up, does it continue to shift oddly? Have you ever dumped any transmission “additives” in there? (don’t, btw).