I have been down that same exact road...at rush hour traffic, on one of the busiest hi-ways in the United States...for the same reason.
I fixed it off the shoulder with a screwdriver and some fluid I keep in the car for just such an incident...
I have Coolflex as well..it took me a while to get it to where I am driving in confidence. I seem to be there now.
Tough to explain in a reply - but if the rubber extension is 4" long, you want to make certain the waterneck or radiator outlet goes into it 2", and the Coolflex hose goes in another 2" - not 3/1 or 1/3 - exactly half (whatever length it actually is). Make sure you tighten down the hose ends every tight - and pull on the SOB when you get done with it - really hard - if it comes apart, redo it, it will never survive a drive. Make certain that the hose ends are inside the portion to be tightening down, not on the edge. Again, it will not survive. It won't be hard to do.
One last thing - you did not mention where it is coming off. Is the Coolflex coming out of the rubber or is the rubber comine of the radiator or water neck? If the rubber portion is coming off the neck, make sure your necks have a nice embossed edge so that the hose has something to grip into. Also, you can take some real course sandpaper around it so the rubber has something to bite - just enough to coursen it in the OPPOSITE direction of the hose - NOT enough to grind anything down.
Lastly - make sure the Coolflex is long enough and it is going into the rubber straight on, nothing angled. I cut mine too short and was going in at a slight angle - thus had to get a new hose....You can see how hi my hose has to go up, to bend straight into the outlet of the radiator. Any shorter and it is going in on an angle, which will never survive.