Boil over

Previous questions involved a starting problem cured by a distributor tune up, a fast idle problem remedied by allowing the engine to warm up fully before driving it, and a lack of acceleration with poor gas mileage which remains unsolved. Now, the last time I drove it was to do a quick shopping nearby and get gas. On the way home I heard it knock going up the first hill for a quick moment, it was very slow going up my rather steep driveway, then it boiled over when I turned it off. This was a first. Usually the temp never goes over half way on the gauge. Could this be related to the acceleration problem we discussed previously which we determined Keep in mind, I did hear a knock momentarily going up the hill. Could the timing chain have gotten so loose it skipped a tooth and jumped time? What else could it be?

Should I stop driving the car until this is sorted out? Is it likely to boil over again the next time I use it?

The only thing that changed prior to the boil over is that I had a mechanic check it out the day before to see if he could determine what the performance problem was. He drove it, took off the air filter, checked the carburetor and a few things on the top end, used a vacuum device to check out some kind of ports for clogging (carbon fouling) and then told me he did not know and the car would have to go through a full diagnostic to find out. Could not he have at least checked to see if there was a delay between the rotation of the crank pulley and the rotation of the distributor rotor to see if the chain might be stretched? I cannot do it because I have no one to help me look at the rotor while I turn the crank pulley.