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All of the items noted by others are possible causes.
If this is a problem that has emerged recently, I would definitely investigate the blocked radiator. When I rebuilt my 142, I had the radiator cleaned (supposedly) and pressure tested. I had a similar problem with the engine running hot when the car wasn't moving. I installed the largest electric fan that I could find and the problem persisted. While investigating the persistence of this problem, I did the hand on the radiator temperature test. I discovered that on the bottom 25% - 40% of the rad, the driver's side of the rad (cross flow rad on 140s) was hot and the passenger side was luke warm. Running my hand along the fins I could feel an abrupt temperature transition right in the middle of the rad. So much for the initial clean-out job that was done on my rad. I removed the rad and took it to a different rad shop and they did a boil out, flow test and repeat pressure test. I put the rad back in and everything now runs just fine.
As a secondary note, I have a B20E with Megasquirt fuel injection. The MS uses the stock B20E temperature sensor on the front of the head for coolant temperature and the dash gauge uses the temp sensor at the back of the head. With the lap top hooked up to the MS, the front temp (post rad clean out) runs very steady under all operating conditions, fluctuating perhaps +/- 1 or 2 deg C as the fan switches on and off. My aftermarket dash temp gauge which has no lag / damping built into it will fluctuate much more, perhaps 5 deg C. When driving steady at 50 kph or faster, the back and front temp measurements are within 1 deg C or so. Come to a stop and let the engine idle and the back temp still climbs up; but, following the rad clean out levels off perhaps 4 deg C hotter than the front. I conclude that at idle, the flow rate through the head is allowing that temp rise. I think it is a design issue as my water pump is new and the engine block and head were tank cleaned before the engine was rebuilt.
There is one last item to consider. In the Volvo head there is a water distribution pipe that runs from the front to the back of the head. Its located behind that large expansion plug on the front of the head. The distribution pipe has holes in it to supply water to different points in the head. The distribution pipe is keyed into the head; however, sometimes the pipe will come loose and rotate, messing up the water distribution and in the worst case allowing the flow to by-pass the pipe with the result that the back end of the head runs much hotter. This is a worst case scenario. Check everything else before you pursue this problem.
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