Your temps that head to the house radiators are really low.
This was posted by willie g, I assume its pretty close to accurate.
average numbers for a typical 3/4 one pipe slant fin baseboard is (rated btu per foot of basebaord)
140 degrees delivery ----300 btu
150---350
160---400
170---500
180---550
190---650
200---700
It is no wonder why you can't keep your house at temp. If your zones are only getting 120 degrees you are only putting out maybe 200 btus per foot!
Can you measure the temp of the owb pipe when it leaves the OWB, if it is 185 and it comes into the house at 143 that means that you are losing 42 degrees from boiler to house, that would most certainly point to water touching your lines underground somewhere. Also with these temp guns they are temperamental about what they are reading against, they don't read off of shiny metal but if you put a piece of masking tape around the area and test the tape you will get an accurate reading. The pex can also be funky to read off of, I read off of an area that is normally covered with insulation, I peel it back and take the reading. Remember to put the gun on the pipe when you take the reading or you will get the gun reading the pipe and other things as well. After we figure out if that incoming temp is really a 42 degree drop or not we can move on. Thanks for the numbers, they show the whole story!