1989 240 DL

So I've been having some transmission problems and I'm worried about my girl.

over the past week, I've been having two different reactions from my transmission.
on good days, she'll shift up with a little foot play on the gas pedal. on bad days, she'll get to 2nd or 3rd gear and just stay there, I'll hit the max MPH for the gear and the engine will just rev higher and higher unless I release the accelerator. I'll play with the gas pedal and it just won't work.

It's been pretty cold in Texas these past few weeks, especially in the mornings. I had heard that the cold weather can affect the transmission fluid and it can have this type of effect if there's too little transmission fluid. So I checked it HOT after driving home one day, and it was pretty close to the MIN line so I went to the local auto parts store and put some in, making sure not to fill it too much. I was hoping that was the issue, but today, it was nice and warm, I just filled the tranny fluid last night, and on my way home from work, I couldn't do more than 40MPH on the interstate. I just couldn't get her to shift up, no matter how much i played with the accelerator. after pulling over more than once, turning her off, then on, and pretty much just stomping on the pedal repetitively she kicked in and I was able to get her back to the condition she's in on GOOD DAYS. I still have to play with the gas pedal (IE: letting up a bit when she RPM's high to get her to up shift) but i know that even this condition isn't optimal.

I'm sure I don't need a whole new tranny cause she's still operational, I just think there's a smaller problem here that I don't know about.


additionally, I know very little about cars, aside from pumping gas, and changing a flat, putting my own tranny fluid in was about as far as I have gone under the hood. But when I bought my girl a few months ago, she was beautiful and I knew that I didn't want anyone putting their hands on her but me. in these hard times of the recession, I've become a bit of a DIY'er and I love learning all this new stuff, so please, someone talk to me, what should i do?