Lights dim when boiler kicks on!
Changing the wire to 12 gauge will probably not fix that problem.
While you are at it, you should make sure all the connections and set screws, including for the neutral and ground wires, inside the breaker panel are tight. If you loosen each a quarter turn and then retighten, you will clean and improve the connection a little. (Do not use tremendous force)
Also uinsnap and replace each breaker (while it is switched off) to clean the contact underneath. If a breaker seems loose you may need to move it to another slot to get an improved connection.
The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.