My 2000 Ford 1.6 lt. Laser Automatic; at around 60 km/h speed, suddenly Gear changes back to a very low gear (engine rev goes up to 4000-5000 rpm) and goes back to normal in few seconds. runs normal at high speed 100 km/h.

no country told , matters! for sure no USA. KM speeds tell that. GUESS Australia????
down under IRON./? or the exported Austrilia to Africa.???

won't rev out (means RPM limited,means low power from engine. so means engine power is lost.
and NOT:
not transmission slip. (none told A/T or stick?)
good post, lots of causes for any kind of engine power loss
I will not pretend to guess it but in terms of physics yes.
engine has mech problem , cam belt slipping then changing back.
engine low power.
does 4LT mean 5liter engine, or 4L +LT MODEL CAR
The BA sub model falcon? IDK
wiki says

• 4.0 L Barra 182 I6 (petrol)
• 4.0 L Barra 240T I6 T (petrol)
• 4.0 L Barra E-Gas I6 (LPG)

seems there are 3 engines used, day1 sold,
gas Petrol
turbo petrol (gasoline)
and LPG (propane) gas.

deep wiki' me.
182 is 6 cylinder DOHC cam engine with pesky VVT valve timing
chain drive cam +VVT
VVT can stick so there is my first physics IDEA on engine problems.
or stick head valve.

the classic list, (means what is tested in order.) using tools.
1: good engine, means all tests pass,and VVT'
2:good spark ever do a tune up in 20 long years,spark + filters.
3: bad fuel, or bad fueling rates. , fuel pump on lasts legs. sure.
4: egr struck open ,even random they do.
5:;random bad wiring to all that.

that is my best, 12,000 miles away and blind, cant see car engine or transmission there.

the rpm limiter will pull back the rpm to limp mode
select low range 1st gear or if automatic select manual 1st gear That will allow higher rpm at low speed

check around gear box repair shops to see if there is a replacement box that has an overdrive gear (5th)
if it is an automatic , the problem may be that overdrive is not coming in and that will drop the rpm 250 rpm and increase the speed up to the 75 mph mark
there is nothing wrong in the box as the rpm and the speed are about right in relation to the diff ratio(not mentioned )
3200 rpm with a overdrive manual runs around 74 mph or 120 km/hr
another method is to put a diff in with a taller ratio
example --current 4.1 back to 3.2

When you rev the engine and does not go over 3000 rpm is this in neutral or when driving in gear? Some vehicles have a rev limiter to not allow rpm over certain point when in neutral gear but is typically not limited when driving vehicle. This sounds like a very low rev limit but not abnormal if you can get engine to rev higher when driving. You call a drifter so truck must be non US market. Is this engine a 2.5 liter diesel 4 cylinder motor. Might just be your top rev and speed if this is the case.

driving in over drive mode actually is what gives the economy as the engine rpms will be around 300 less than direct drive . Lower rpms = fuel savings
OD should come in around 60 klm/hr and drop out at around 50 klm/hr. When you are accelerating hold the pedal in a set position and watch the rev counter (rpms ) and each gear change you will see the rpms drop and then build back up as the vehicle speed increases At 100klm/hr the revs should be around 3200 rpms or less but if over ( say 3500 rpms) then may be the OD is not coming in. One way to test a box operation is to find a speed shop with a dynomometer and test the gear selections against loads and engine rpms

I would get a transmission service oil change a clean and flush sometimes debris build up if the trans is slipping you may need an overhaul

your trans is not shifting into over drive or your loclup is not working, try pulling codes from it or take it to a trans shop for a diagnostic, may get expensive good luck

This may apply. ...............................

Carry out the adaptation drive cycle road test:

Note: This process must be carried out in normal mode (not sports mode). On a flat road, the transmission fluid temperature must be above 60° C (140° f) and below 100° C (212° f) (vehicle at normal operating temperature will ensure that the transmission fluid temperature is above 60° C (140° f)).
9. Accelerate from rest with light throttle (do not exceed the upper rev limit in appendix 1). 1-2, 2-3, 3-4 shifts must occur with engine speed between the following limits:.
Note: Do not move the accelerator pedal during shifts.

• 3.0L VIN ALL 1800 - 2000 rpm
• 4.2L N/A Prior VIN M96321 1450 - 1600 rpm
• 4.2L N/A From VIN M963221550 - 1700 rpm

10. Continue to accelerate gently to 50 mph (80 km/h) so that transmission shifts into 5th gear.
11. Go into overrun and let the transmission shift down into 4th gear without braking.
12. Brake gently to a standstill and hold footbrake on for at least 15 seconds.
13. This process should be carried out at least five times.

Page 2 of 3 Bulletin Number S307-17
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Change the Engine speed sensor. This can sometime cause your dash to do all sorts of weird things when they fail, such as setting the low fuel warning off as well. strange but true ! good luck

There is a speed/shift sensor you could change, but if you haven't changed the fuel filter in a while, i'd go there first. Sometimes, in fact, often, plugged fuel filters can cause the engine to starve for fuel, and make it seem like the transmission is the problem.