Car feels bumpy?

[Martin Suryadarma]

Hi listers,

I have a question about my 89 Audi 100. Lately I notice that sometimes,
only sometimes, my car feels like it's riding on bumps especially in
low-speed. Not huge bumps, but more like frequent small bumps. In highway
speed it never happens, only during very slow speed like in city traffic
driving. Again this doesn't happen all the time. 

I'm not sure if it's engine related because the tachometer stays normal 
and I don't hear anything funny either. Power is also there. Could it be
axle-related? Suspension? 

As always, any ideas or info is greatly appreciated. Thanks in advance.

Martin