2000 leganza automatic trans wont engage at times

[wajidameer]

I am having Leganza 2000 since almost five years, not vary long since I bought I had a problem in automatic transmission it started with car shaking when I bring car to a complete stop, I had to replace transmission as mechanic said it cannot fixed otherwise.
This transmission worked for few years then last year I had a problem with gear shifting from second to third, it use to come and go mechanic told me it could be related to heat (as I live in Saudi Arabia and temp go up to 50+ C), so he disable the tem sensor and now fan runs all the time. but later on the problem comes and goes, some time car won’t move a meter and some time it works alright for months. Now mechanic tells me to replace it.
What I don’t understand if there is something wrong it should just stop working, how come sometime it starts working fine and some time it just don’t engage at all.

[Trackerrrr]

Have you checked the fluid level? That's the easy thing to check.