tranny slipping into n Options

[Malhar95]

Posting this for a buddy:
It's a 7a auto 1994 (corolla though)

The tranny keeps slipping into n after driving it,
It did before 2 weeks ago, checked the fluid it was low, then after a qt it showed too much but when we pulled the drained plug it pulled only a qt out (so dip stick might be bad) then it was normal driving fine

Today the car went into neutral suddenly, fluid shows fine.
Should we drain and put new oil in since dip stock can't be trusted or could it be something else?

I remember similar thing happened to my celi but it was something to do with shift cables..

[Malhar95]

problem is still there. the shop doesnt know whats wrong, "we have to open it up and check whats damage and rebuild the tranmission,then get u a qoute estimate for repair. if u decide not to fix it, its 300 to get the car back together"

so i just got the car home. is there any things i can check for myself, i was gonna replace the tranny myself eventually but until then, and suggetions?
torque convertor? (reading some stickies, sounds like u can replace the torque conv. without removing the tranns. is that correct or am i misunderstanding? its in the auto-manual sticky when mentioning the bolts to take the torque conv off)

any tips greatly appriciated