Transmission slippage...

i've been stressing over a "minor" problem with my automatic transmission in my 91 geo metro. very rarely, and randomly at that, the transmission slips. i had it flushed and then a week later i drained it and replaced the gasket and filter and then refilled it with nice new fluid. it runs great but on a rare occasion it will slip and when it does it will spend a little bit of time to quit. but like i said it does it so rarely i wonder if something just needs adjusted. i did notice that when it is in DRIVE, it can be shifted to NEUTRAL without unlocking it. however, on only some of the gears it does this, but others it won't budge like it is locked. is this my problem that causes slippage or is it something else? i appreciate any help

by the way i have driven it hard the last few days and it has been responsive and "slip" free. that's why i am so frustrated!!!

Being able to shift from DRIVE to NEUTRAL without unlocking is normal. It should however be locked from REVERSE to NEUTRAL. Have you check your transmission fluid level and what type of fluid did you use?