88Sunbird no heat

[windmill68]

I have an 88 Sunbird 2.0L no turbo TBI "K".
The engine temp is doing strange things. The engine will heat up to 60 Deg C then quickly drop to about 45 Deg C. It will keep doing this till it levels off at about 50 deg C. The head gasket was changed two years ago, but it was doing this before the gasket was changed. It's actually been doing this for many years. I changed the thermostat and it made no difference. In the summer time, if I let it sit and idle, it will gradually hit 85 deg C and will regulate nicely at that temp. But once I start driving, it will drop to 60 deg C.
It doesn't make any sense.
Any suggestions?
Thanks.