Woe! Quite a few responses since I was last able to post. Thanks all for your contributions.
OK I give up - my computer is a custom built:
Asus P4C800-E dual processor (not dual core) mother board.
2 X 2.6 GHz Pentium 4 processors
2 X 120 GB hard drives
2 GB RAM
Matrox Millenium P750 video card
3 X monitors
Fresh XP Pro installed (not including the recent XP updates that have been problematic)
Updated drivers
Updated Java
In all other respects it runs well.
Again - I have previously had fifty charts running along with other programs without a problem. Monitoring the CPU's shows no sign of stress or that they are even breaking a sweat. The RAM only reaches less than half its capacity. The CPU is not hanging, otherwise it would also show in other programs being run simultaneously, and it does not.
I contacted IQFeed and they walked me through a thorough dianostic of the feed and it is fine. They also checked the internet connection from their end and it is excellent. Also the problem exists whether I use IQFeed or IB feed.
I have played with the feed update speed with no change to the problem.
There are no additional symbols updating in the background. In fact the stock data file has been emptied before each test today. Program or computer overload is not the problem.
Jerry from QT came on my computer today and did some tests and took some QT files to play with. Something seems to be corrupting the SDATA file or the input to it or its output and processing of the data by the QT program. Hopefully Jerry can come up with an answer.
I also intend to play with a couple of Windows Services related to data movement through the OS on Monday to see if they are related.