Hey Miguel

interesting reading about these heater valve issues, as I'm thinkin' I'll have to replace my valve at some point in the near future. it's been doing the (binary) off or full on thing for about three years... though it's not leaking heat when off (yet, and knock on wood).

As for the heat coming through only when accelerating, my theory is that at higher revs, as the water pump is spinning faster, there's higher water pressure hitting up against the valve, and some is able to squirt through. thus you're getting heat when accelerating. What happens if you just hold the RPM's up while not accelerating? i'll bet you still get heat. then at the lower rev's, there's not enough water pressure to force a detectable amount of hot coolant through the leaky valve.

just my opinion, for what it's worth,
good luck with it though.
link