Auto tranny slip? R33 GTST

[BLUD]

Hi crew

I have just bought a 95 Skyline GTS25T Auto. I have noticed on the highway at about 110kph that the auto will drop down to 3rd suddenly for no reason (not under accel.) then fluctuate a little up and down. I thought it may have just been overdrive playing up, but it also did it a little from 3rd to 2nd when cruising at 80kph.

This happened on flat roads and going DOWN hill (not up - I would expect sometimes it to shift down climbing a hill)

My first guess would be tran fluid. Could it be due to cheap thin fluid? Should I go for some high qual synthetic, or get it checked out?

cheers,

Brett