I'm working on a 2 year long project that currently has about 180 tasks. The project involves several engineers and we are trying to keep our spending to a minimum by avoiding the purchase of MS Project licenses (it would cost us in the neiborhood of $10,000). Because of this I have been looking at several open source PM programs, but all of them have serious issues. Here is a list of the ones I've tried:

1. GanttProject
2. OpenProj
3. OpenWorkbench

Here are the problems I have had:
1. Critical path is calculated incorrectly.
2. Problems with the tasks suddenly moving forward several days.
3. Problems with earliest start date.
4. Problems with weekends and vacation days being ignored.
5. Various resource management issues.

Does anyone know of any program that is a true replacement for MS Project? In other words, something that can calculate task logic and critical paths correctly?