At hwy speeds auto trans thuds then sticks in third gear

"I am driving to work this morning on the interstate at around 60-70 mph when I here a thud come from the car. I look down and the check engine light has come on but the car still sounds and operates normally.

I noticed that the rpm has gone up so I slide over into shiftronic mode and notice that I am in third gear and can't up-shift to fourth. I wasn't far from work so I drove in and noticed that I could not shift from second to first.

I backed into the space at work to see if i still had reverse and I do. I turned the car off and back on and i was in first, so i took it through the parking lot to test it and i was able to shift through the first three gears again. i wasn't able to get up enough speed to try fourth.

I hear no noise from the tranny. Fluid, although in dire need of changing, did not have any artifacts in it.

I am confused as to what is causing this. Any thoughts ?"

I am having the same issue as this guy, there was never a solution posted under this thread, so now I am creating a new thread and asking for help.

Best,
Lou

PS When I turn the car off then back on again I regain fourth gear, however it tugs a few times then I lose it again in a matter of minutes. I think that the transmission being hot plays a part in it. Also I filled the fluid level so low fluid is not the cause.