i've a lot of 3000L installed and in all of this the ethernet speed (with around 20mt of cat5E FTP) flapping from 1gbps to 100mbps then back fixed to 100mbps.
I've changed 3000L with a F300 and link is 1Gbps....
The Ethernet subsystem is pretty much similar between F300 and ePMP3000L. The main difference that ePMP3000L had a dedicated grounding lug while F300 don't so I guess it is something about the grounding on site.
Please check the following thread for a good discussion on grounding: