Volvo 240 thermostat?

Got a 1988 Volvo 240 DL Wagon. With 293000 miles on it. Back in June the engine temp (according to the dash) started climbing up when I would drive- I discovered I could get it to go back down by quickly shifting into neutral and turning the car off and back on- though this was still annoying since I would have to do it several times on my 15 minute drive to work. I replaced the thermostat, and the problem went away. That is, until this past week (October). The same thing is happening; I replaced the thermostat again, but this time the problem didn't go away. Any suggestions?