I replaced my thermostat about six months ago.
Yesterday, I found my temperature gauge drifting down as I picked up speed on the highway. I assume the thermostat has failed in the open position.
On the other hand, I'm surprised, given the interval. Could I have a bad coolant temperature sensor?
I have installed two Saab OEM thermostats, and both have worked perfectly--even when I use Tech II to tighten the band in which the gauge stays horizonal. It always stays horizontal.
On the other hand, I installed an OEM equivalent Motorad thermostat (Saab is the only fitment for this model) and the gauge would droop as you describe.
Oddly, the Saab OEM thermostat was made my Motorad. However it had some detail differences, and it didn't do that.
I have only changed one temp sensor, and that was because it was bad (the pigtail broke off). A sensor would read consistently off, but if it goes to horizontal and stays there if you let the car idle, then it's probably reading correctly.
TL;DR: get Saab OEM thermostat (good luck, hope Orio hasn't repackaged something from Proparts :nono

, install, and see if you still have problems.