Sounds like a standard Spanning Tree issue. Enable portfast on the client ports.

https://support.dell.com/support/edocs/network/pc33xx/en/CLI/J0926a00/spanning.htm#1018036

I enabled portfast on a single port for a problem workstation.  It did not help.  The workstation NIC is an Intel Pro 1000.  I've updated the driver and tried setting the link speed to 100/full and even 10/half but nothing helps.  The same workstation plugged into an HP Pro 10/100 switch works fine.

What am I missing?

Your original problem sounds like a spanning tree convergence problem as Greg said.  Spanning tree will take up to two minutes to converge on some network so when you first plugin you might not be able to pass traffic immediately until the spanning tree protocol converges.

Now when you start experimenting with the port you mentioned you tried to configure the port to 100/full, 10/half, etc.  This is not a good idea!  Whenever you do things like changing a port configurations from "auto-negotiation" to force and changing settings like duplex or speed you must be VERY careful to make sure both side of the links are configured properly otherwise you will end up with very weird unpredictable behaviors.

If one side of the link is configured to be auto-negotiation and you configure the other side to force 100/full the auto-negotiation side will (by standard definition) drop down to half-duplex which may cause your connection to either exhibit very poor performance or just fail completely after awhile.

You should either configure both sides of the link to auto-negotiation (recommended) or if you do force the link then make sure you check both ends of the link and make sure they are configured EXACTLY the same!

So back to your original problem, what you described is typical behavior of spanning-tree convergence.  As Greg suggested you should enable port-fast on the switch.  It maybe that you did not configure it completely if you said you did try enabling it.  If you are still having this problem, first make sure you check the port configuration to verify you don't have the port-mismatch problem described above.  Then try enabling port-fast again and if you still have a problem then post your configuration for the switch here along with a description of your network topology and maybe we can help you spot the problem.

Cuong.