Overheating problem

Low-speed overheating is usually an airflow problem, not a waterflow problem; at higher speeds the car's forward motion pushes more air across the rad. If the rad were faulty you'd tend to see overheating at high speeds first, then at all speeds. Check to make sure your rad fan(s) operate correctly. Check to make sure the front face of the A/C condenser (in front of the rad) isn't clogged with bugs and dirt. Then pull the rad and make sure its own front face isn't clogged.

There is nothing such as a "summer thermostat" (no really, it's true; I checked the calendar and it turns out we aren't in 1953!). There is one—and only one—correct thermostat rating for a vehicle such as this with computerised engine management, so putting back the correct thermostat is going to be on your to-do list. If you installed a lower-temp thermostat because this car wants to overheat unless the weather's cold, diagnose and repair the actual problem (it's not a too-high thermostat rating) and then install the correct thermostat.