1988 integra auto shift 90,000miles:
When the engine is cold, say overnight, when I shift the gear form
Park to Reverse, there is a very big jerk(in fact the whole car jerk).
The same thing happens when I shift from D to R and R to D backforth.

But this "sharp jerk" is a lot less serious when the car is warm.

I was thinking if this is due to transmission fluid being low,when the
car was not driven overnight, the fluid levels drops down and some of
the gears are not coated with fluid. Any ideas?