I have an automatic 88 300e. When I'm driving along, the engine revs suddenly increase (without my help) but I do not speed up. It seems as if I'm in neutral. I happened to me twice today in 4 trips

When I drop the transmission into the next lowest gear (3), it seems the tranny reengages. This problem does not appear in gear 3, the gear just below D, only when I'm in Drive, and only when a gear change ought to have occurred because the appropriate revs were met.

The problems also disappears sometimes when I take my foot off the gas and let the revs drop.