Faulty Thermo??

My 87 has for the last week giving me some concern about how hot the gauge tells me the water temperature is getting before the thermo opens up. It goes within two lines of the top then opens up and goes to about the thermo rating of 190*. Since I got my radiator replaced and this thermo installed it has been going only about one line obove the thermo temp rating. Today I noticed the gauge fluctuating between roughtly 240* and 190* slowly during driving in about 50* temperatures. Normal?? When I let it idle the water temp actually goes down to normal but then when I drive slowly it slowly climbs up to almost 240* and then goes back toward normal. On fast drives over 45 mph it never climbs up high and sits at about `190*! Any Ideas?? I think it is a bad thermo! Any easy way to test my idea??

Thanks, my 87 has a fan clutch driven plastic stock fan using underfrive pullies and an EBrock pump. Think it could be air in the system after a year with no problems? Yes I am using a no name NAPA thermo cause my mechanic did not like my 160* lifetime Saleen thermo. I will try a new Mr Gasket thermo. Yes I am guessing on the temp with my stock temp gauge so the actual temps could be off. I judge from where the thermo has opened up with past good thermos. I know I should get a real gauge!!