We have a large (1200+ hours) website that has a lot of technical debt. This is mainly caused by the following (usual) reasons.

1. Multiple programmers who come and go during development.
2. Change of specifications during development.
3. Numerous added functionalities added (in a short time).

The customer wants alot of new functionalities, and that basically comes down to working on this project weekly for 10+ hours.

Due to the technical debt, we spend A LOT of hours fixing or investigating problems, that usually find their origin in one of the following:

1. A shameless, silly bug that makes people cry.
2. A new feature results in the above because we hadn't foreseen all the places the new feature would have an influence.
3. Some other problems we have faced (f.e. server migration, upgrades)

We have issues daily and we have tried to following things to put this to an halt:

1. Created technical documentation regarding the import, payment and general working of the website.
2. Have meeting at the start of the week - discussing the current issues or improvements and how they should be tackled.
3. Have a test-plan. Programmer A test B, B tests C and C tests A. Then our Project Manager will throw in some tests. Regarding the impact of the feature we throw it on a staging environment and let the customer check for itself.

The problem is that the problems keep happening...and somehow we can't get a grip on it. New features still cause bugs, and old bugs keep saying hello. Somehow - perhaps due the size of the project - we can't seem to get a grip on this project.

I assume there are alot of programmers working on larger projects then this. That is why I come to my question:

What can we do, or what do you do to avoid these problems on large projects?

We have a large (1200+ hours) website that has a lot of technical debt. This is mainly caused by the following (usual) reasons.

1. Multiple programmers who come and go during development.
2. Change of specifications during development.
3. Numerous added functionalities added (in a short time).

The customer wants alot of new functionalities, and that basically comes down to working on this project weekly for 10+ hours.

Due to the technical debt, we spend A LOT of hours fixing or investigating problems, that usually find their origin in one of the following:

1. A shameless, silly bug that makes people cry.
2. A new feature results in the above because we hadn't foreseen all the places the new feature would have an influence.
3. Some other problems we have faced (f.e. server migration, upgrades)

We have issues daily and we have tried to following things to put this to an halt:

1. Created technical documentation regarding the import, payment and general working of the website.
2. Have meeting at the start of the week - discussing the current issues or improvements and how they should be tackled.
3. Have a test-plan. Programmer A test B, B tests C and C tests A. Then our Project Manager will throw in some tests. Regarding the impact of the feature we throw it on a staging environment and let the customer check for itself.

The problem is that the problems keep happening...and somehow we can't get a grip on it. New features still cause bugs, and old bugs keep saying hello. Somehow - perhaps due the size of the project - we can't seem to get a grip on this project.

I assume there are alot of programmers working on larger projects then this. That is why I come to my question:

What can we do, or what do you do to avoid these problems on large projects?

Minor edit, extra info:

1. We use version control (SVN).
2. We have DTAP development process.