Temp gauge, heater, thermostat problem

[Doyleywood]

The temp gauge on my transit has been yoyoing from cold to the usual halfway point,it will do this all day long, this is way any past the usual warm up time, it’s always only taken less than ten minutes to reach working temp,when the gauge is cold the heaters cold, when it’s in the usual halfway point it’s fine, nice and warm. This is an intermittent problem if I stop turn the ignition on and off this will sometimes cure the problem, I’m suspecting the thermostat/temp sender? Has anybody changed a thermostat or replaced the whole housing switch and all ? Is it a job a home machanic can do? Is it easy to access? Thanks for any info, help. 15 plate 2.2 125 128k miles