Guys, I have a low mileage 300d from '81 and most of the problems I encounter end up being minor fixes, so I'm wondering what the simplest explanation & fix would be for my transmission to flare a bit between 1 and 2 and then between 2 and 3.

More backround:
-Problem just started a few months back
-Doesn't happen on a cold start. This is only after driving a couple of minutes when things warm up.
- If I let off the accelerator a bit as it's about to shift, I don't have the problem
- Problem is accentuated the faster I'm accelerating
- Shifting goes like this: Feels like it stays in 1st gear just a tad too long... then flare between 1 and 2... then flare between 2 and 3... Then 3-4 seems to happen too quickly.
- Changed my transmission fluid/filter and this didn't solve the problem
- Transmission fluid level may be a fraction of a hair too high, but no foaming or signs of overfill that I know of... Unless this slipping can be caused by a slight overfill. Does it have to be laser precise?

Again I'm looking for the easy fixes first.

THANK YOU!