I've been having a similar problem.
Problem: ImgBurn is not writing to DVD-R at the selected burn speed. Below is a summary along with all the regular burn log, post-burn side panel and .ibg file. On my next burn I will post a debug log and pre-burn side panel.
Summary: I clicked on "Write Img to Disc." I selected write speed of 3x. as this is supported by my DVD-R media. The results of my burn are as so: Average Write Rate: 7,439 KB/s (5.4x) - Maximum Write Rate: 10,917 KB/s (7.9x). I cannot verify that my hardware supports 3x write speed, however I have burned many successful writes at 3x write speed using UltraISO. Then again, UltraISO doesn't show real-time write speed, so this statement may not be entirely accurate. My next burn will be 4x. I didn't have any problem at 4x, 6x, or 8x with Imgburn, but I have not tried those speeds since I updated to the most recent version; only 3x.
Thanks!
Burn_and_Verify_Log.txt
Side_Panel_Device_and_Media_Info.txt
BOB_FUNK_3x_....CMC_MAG._AM3.ibg