Is this a bug?

[Kiah07]

CGMiner 3.6.6
Catalyst 13.10
HD6870

Cgminer usually reports ~300kh/s with
cgminer --scrypt -o stratum_address -u username -p password --gpu-platform 1 -d 0 -I 15 -w 64 --thread-concurrency 6720

Issue:  Sometimes after stopping and restarting cgminer the speed only gets back up to ~14kh/s.  This has happened to me more than 10 times this week.

Solution:  Whenever I close firefox, cgminer increases speed from 14 back to normal 300 kh/s. 

Does anyone have any reasons for this?

[deepceleron]

Disable flash and other plug-ins, flash videos will auto-drop the GPU speed.

[Qbeczeq]

Firefox probably uses GPU drawing too, or maybe one of plugins as mentioned before. Try install Chrome/Opera/Safari and look if these explorers also doing the sam problem