I've never had problems burning CD's, and I always use 40X (fastest my old burner supports). How old is this computer (what are the system specs?) because older computers have a lot of trouble running Ubuntu in live-CD mode (which uses tons more resources...well, the same amount of resources, but there are much less resources available in liveCD mode...than when it's installed). A PC with 256MB or more RAM, Pentium 3/Celeron/Athlon 450-500-600MHz minimum (may go lower, but I wouldn't recommend on anything lower than these).
If your PC has 2 CD-ROM drives, try using the other one.