ignition problem at 4800 rpm

Hi,

This is a great site! I hope someone will advice me on this one:

I have had my car ( Audi V8 Qattro, 1989 model) since the beginning of the 90-ties. It has been great and is in good condition. 200 000 km so far.

Now I have a problem with the ignition (?). At about 4800 rpm acceleration stops and the engine sounds disturbed. I dare not push it more since I understand there is some malfunction. What to do?

Best regards//C