It sounds as if your clutch is not fully releasing under some conditions. It would be wise to bleed the clutch and see if it helps.
It may also help to change the trans fluid. The most popular mix is the Mobil 1 "cocktail" as outlined in the FAQ. It also helps to use dino fluid (Mercon spec) with the same 2 oz of Ford friction modifier. I personally use Chrysler spec ATF+3 which already has a higher level of friction modifier. If the problem is from the clutch not fully releasing at times, the fluid change will not make much if any difference for the exact problem you are describing, but the overall shift quality will be greatly improved. It is well worth the effort.
