drop in RPM is most probably due to sticky IACV,,,
no need to dismantle TB to clean it, there is a pipe connected on the upper part of air intake pipe, u can spray carb cleaner in it while the car is running, engine will shudder and shut down, dont worry start again and rev it a bit.. there is a hile in the neck of TB, a bit tricky to reach but accessible use nossel to spray carb cleaner in it as well, also drop an oil of PS fluid in that hole after wards..
are u running it on tap water? which is not a gud idea, atleast use the guard coolant, green one, which is help u in tracing any leaks in system.
wat is condition of the radiator cap? u can try replacing it i guess 1.3 has 0.9 rad cap.
do check the brim of radiator for any deformities.
the pipe going to wards reservoir bottle must be air tight means it must not suck or leak air during operation, the cap on bottle should lock tight, the coolant level shud not exceed max. mark and pipe to remain submersed in the liquid.
Tstat valve shud be replaced with a brand new one after testing it in boiling water, i guess it takes 83 or 89 and new one is easily available for 1200-1400 for original part.
reducing a cup of water in mere 50 km drive is too much, must check where is that water going? is the expansion reservoir over flowing? do u hear steam entering expansion bottle when u turn of car?
is there original copper radiator installed? wats it condition? repair history?
also do check the coolant lines going to IACV in TB, the tend to leak with age, if the system is sealed then u probably hear a hiss made by escaping steam from cracks in rubber pipes.
also check the pipes going towards heater core and check the carper under blower assembly to find any dampness due to leakage.
u can find the leaks easily if there is coolant in system.