Pull your Denise chip and clean up the pins and socket connections. If the socket cups disintegrate (as some on mine did) remove the socket and either put in a replacement socket or do what I did, which is solder the denise right to the board. If you still have problems you can patch in by soldering some signal wire to the appropriate spot to the leg of the denise.
see:
http://www.amiga.org/gallery/index.php?n=1326http://www.amiga.org/gallery/index.php?n=1325
