Tony.... The only thing I can think of at the moment that would affect the heat range of separate cylinders are the "Rubber Water Deflectors" that are inserted into the water passageways at various places to control the flow of the water. These would be pictured in any parts manual pertaining to your engine.
In time they do have a habit of swelling/expanding sideways... and when that takes place, the water flow is somewhat blocked, sometimes completely. It is unusual for the swelling to just affect two cylinders but it has happened.
Unfortunately the only way to inspect, remove, replace, them is to remove the cylinder heads... BUT... if it were me, that's the route I'd take. The following is taken from my database and may be of use to you.
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(Water Deflectors)
(J. Reeves)
Water deflectors are actually lengths of 3/8" outside diameter rubber hose, installed between the top cylinder and any cylinder beneath it, and also between the bottom cylinder and the block. The purpose of the deflectors is to have the water follow a definite path around the cylinder walls. Unfortunately the deflectors between the cylinders will at times swell sideways due to either a previous bad overheating problem, or simply due to age and salt corrosion. This causes a water flow restriction which usually allows the water to cool sufficently at low rpms but not at the higher rpms.
This hose material can be purchased reasonably at any automotive parts type store if you care to make your own, or you can purchase individual deflectors at any Evinrude/Johnson dealership at a somewhat higher cost.
Removing and installing them can be a hassle at times, but not always. I use a sharply pointed scribe with about 1/4" of the tip bent at a right angle whereas I can reach in, jab the tip sideways into the rubber, then yank it out. It's necessary to clean the seating surfaces where the ruber contacts the block with a small rat tail file to eliminate salt deposits etc. When installing the new rubber deflector, coat the deflector and the metal surfaces with WD40 which will act as lubrication to allow it to go in as easily as possible.
Make sure that you insert something into that deflector area before cutting and installing the deflectors if you make your own so that you will be certain that they are the right length and also that they will be seated properly. Usually a very small amount will be left extending about the block sealing area..... simply cut the excess off with a single edge razor blade.
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