Troubleshooting a soufflé that collapses during baking
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.