When my 93 300E started acting the same way I took it into a shop and they said it was fine after a test drive, suggesting a fluid/filter change. Then they opened it up and clutch material fell out. I had it overhauled at a price of $1900. Check the fluid first, low level can cause slippage, and as suggested prior, check for the smell of the fluid. Techs can tell you more about this, but often an adjustment to modulator pressure can help, or changing the shift points via the adjustment on the bowden cable (something I am currently dealing with) since your problem is at low shift points. If it was me I would do a fluid and filter change, especially if you are over 90K miles and/or it has been awhile. If been a while it cant hurt. If you DIY you can check it, or have the shop check it while it is open for problems, should be under $150.