I think I know what is going on here. The coolant system is working TOO well, so the question is what could cause that. If a new thermostat is failing open, and the old one was warped, something is subjecting the thermostat to stress. What if the bypass is clogged, and high pressure is thus forcing open the thermostat? The bypass on a stock 2g goes over the exhaust mani and into the heater core, then connects with the radiator outlet. On my car, the turbo coolant lines are inline with the heater core. Perhaps there is a partial obstruction in the turbo/heater core coolant line, causing high pressure at the thermostat, warping the old one and pushing the new one open all the time.

The car hits normal operating temperature if just sitting, because there is no air flow until the fans turn on. But when I drive, temps drop to the 140s-160s, b/c flow is always going on through the radiator.

I will have to find a shop that can do a high pressure flush of the turbo/heater core and test this.

2gnt: '99 RS-T, killed by a toyota, pending rebuild...
Daily: Volt
Daily #2: '99 EVG ebike- STOLEN by PEDRO