BeetleJuiceDNL

I just wanted to say before i continue, I'm 21, Female and this is my first car I've owned for about 3 years ( not that any of this is really important)I'm starting to want to learn more about cars and how to fix my own problems and save money instead of relying on a mechanic. So far I have repaired my busted front bumper, repaired and replaced R headlight housing and bulb, and Replaced an 02 censor. I'm having issues with my transmission shifting gears in cold weather, but it runs like a dream in the summer. This was a problem last winter but wasn't as bad. Before, while accelerating and merging over on high-way, it wouldn't use the 4th gear at first but then it would shift gears after driving for a bit. Does the same problem now, but won't change gears at all or takes 10 mins+ to shift itself. sometimes has issues shifting 2nd gear to 3rd. es300 is 206xxx miles, 1999 model, and the transmission was rebuilt July 2016. I'm afraid this might have something to do with the previous failing but i want to try and fix the issue myself but can't figure out why this is happening or where to start. Coolant meter is always where it should be, oil is changed religiously, 02 sensor bank 1 sensor 2 needs to be replaced. 4000 rmp at 75mph... (o/d drive is not on). Sometimes when braking hard or getting off high way, doesn't accelerate as it should, almost as if it skips 2nd gear. Accelerating 1st to 2nd at stops goes back to normal if I restart engine. Over all, why is this happening and how can I fix it? Only happens during cold temperatures below 40.