Troubleshooting a soufflé that collapses during baking I am trying to make a cheese soufflé to surprise my girlfriend on her birthday (24 Jan). After several cooking trials, I still face a key problem: my soufflé collapsed after baking for 25-30 minutes. 

What did I do wrong? Here are the steps to the recipe;


boil equipment
grease butter to the renakin, place into the freezer
separate egg whites (no yolk mixed)
add a little salt to egg whites
add a little sugar to egg whites 
using a hand mixer, whip the egg whites adding more sugar during whip
melt 40g butter (100C)
Whisk 40g all purpose flour into melted butter
pour 250g milk slowly and whisk util it mixed
wait until it cools down
re-grease butter to the renakin, place into the freezer
beat the egg yolks to a creamy consistency, then pour into the milk mixture
add 1/3 of the mixture to the base, continue to add the whites folding very gently.
Preheat oven to 200C (5-10 minutes)
pour the mixture into the renakin
add the cheese on the top
bake at 170'C (with heat select="lower")


After about 25-30 minutes and a 2cm rise, the soufflé collapses prior to removing from oven.



updated 19 Jan Evening :

I do


buy better hand mixer
use more flour (50g)
move egg whites whipping process to last steps
reduce whipping time to 5 minutes
bake with heat select="lower" about 15min after that change to "upper and lower"
sit in front of oven and see the soufflé every moment.


result:


at 20mins: the soufflé raise about 3.5 to 4cm (from 5cm height renakin)
around 25mins: it collapse about 0.5cm, so I immediately remove it from oven (T-T)
after than it collapse very fast. (I guess it is common)


Thank you very much @jefromi and @KatieK for your comments and links. I plan to buy oven thermometer this morning because I have no it yet. 



updated 21 Jan Lunch :

Yesterday I found problem on the oven temperature. When I set temperature to 170 C, real temperature is 150C or below (because it keep going down). So today I try to keep in range of 160-200C and no collapses now. 

Thanks you very much to everybody again for kindly help.
There are two likely causes of soufflé problems.

1) Egg foam. Be sure that you're whipping them to peaks (soft peaks, as I recall) when you first whip them.  Then, when adding the 2 mixtures together in 3rds, be sure that you're properly folding in the whites; this is a very specific technique, and easy to over-do.

2) Oven heat.  Use an oven thermometer to check your oven's temp - it's probably wrong.  Put some extra mass in that oven (like a baking stone or brick) to reduce wild temperature swings during baking.

But, as in the linked questions in the comments, all soufflés fall to some degree.