Stuart, I have read your comment about a half dozen tiems and don't see what is causing your difficulty. I have done many dadoes and sliding dovetails and have not had the problem. Mine are generally wider (3/4") and deeper, maybe 1/2" and the bits are sharp. I generally route a path right down the center with a straight bit, sometimes an up cutting spiral about 3/8" sometimes a 1/2". The first pass cuts away the bulk of the wood, taking some of the load off the dovetail bit and leaving a path for the chips to escape. Usually do this on a router table without dust collection. The wood is dry, acclimated to the shop, but air dried.
Sometimes do it with a router with a guide bushing and a home made dado jig. When I do, I use two routers, one straight bit and the other dovetail.
Just babbling on here, trying to imagine what we do differently.