I would suggest changing the transmission fluid and filter before adding any sort of additive (but I am generally no fan of "miracle in a bottle"-type products, especially when used in lieu of proper maintenance). The fluid may seem fine, but it is overdue for a change, and old fluid and/or an occluded filter can cause all sorts of strange problems. BTDT - Basics first.

Is it only slipping when in gear, or as it shifts? If you get any slip/flare in the 2-3 shift, you may want to change K1 accumulator spring whilst changing the fluid and filter. I'm unsure, however, if a tired spring would account for such an intermittent problem.

Good luck,
Tom
'82 300D 317k
'96 Passat TDI 177k