Guys. Little question. Whats the reason of delayed engagement on first to second gear when stop and go traffic jams on working temperature?

The problem is like this. Gears are shifting smoothly from the start. then after 10-20 miles of driving the first gear neds RPM 3000-3500 to catch second gear.

This only happening some times and after 10-20 miles of driving and also when driving in stop and go traffic jams
(hope after driving 10-20 miles, its coming on the maximum temperature that time of drive tranny in)

Tranny was rebuilt recently and that problem was there before and after rebuilt too.
And no error codes given.

One of my friend told me the issue is like ths.
When the tranny comes to the maximum temperature, the oil viscosity getting low and act like water. and it causes the first to second shifting hards because of watery like oil. So putting external oil cooler may fix this problem because the oil cools every time.

Is that true?

Need information on this and what you guys think about this problem would be? has anyone faced this kinda situation before?

Car Euro Accord CF3 - 1.8L Auto 4Cyl