This is just my opinion and I just went through this with my Johnson 4hp.
Just bite the bullet, buy a waterpump impeller and kit and change it. It isn't a hard job and the parts
aren't expensive. Try not to remove the shaft from the lower unit unless you have to. The seals
around the shaft can be damaged if you aren't careful.
That cured my intermittent cooling
Your impeller has probably very little flexibility in the vanes and that's why its not pumping continuosly. If it has been more than 3 years since you've changed it it probably is time for a new impeller.