dropping out of 4th gear hot

[samjesse]

Hi

My 2005 V6 drops out of 4th to 3th gear and back again "jack hammering" on the freeway, but not when it is a very cold day when it runs like a dream. No codes or check engine light on. fluid levels OK.

Any ideas?

[MY VZ]

are you going up a hill or slight incline?

[samjesse]

Yes that is right. and After a 10 min. drive.