Sticky thermostat or spring?

[chubcobear]

Once in awhile (one time in 4), in my 1987 Johnson 90 hp (J90TLCUR), the thermostat or spring sticks and the heat alarm comes on at about 170 degrees and goes off at 190 (plus or minus) when the thermostat opens. After that it runs fine (at about 140) all day. My mechanic replaced the thermostats last season and I am thinking I have to go in and re-do it again myself. Or, perhaps, this once-in-awhile sticking (if it is sticking) is normal for this engine? I have only had it for 2 years. Any ideas or experience? With the info I have gleaned here on this forum, I am not afraid to get an OEM Manual and try it.
Tom C. Bear