good day tsikooters!

I just bought this 91 volvo 460 automatic as a project car and i am having 2 issues to diy at the moment.first one is overheating, service history reveals that the previous owner brought the car to the dealer for repair due to overheating while at idle.they changed the thermostat,fan switch,and coolant.

That was 8 months ago and right now it's still overheating at idle.the temperature needle goes way up near the red line after 15 mins of idling.I drove the car on the motorway for 20 miles and the needle went up and remained on the same spot.This morning i checked for coolant leaks but there is none.No sign of oil on the expansion tank or coolant on the engine oil and no fluid loss.Hose appeared to be in good condition and no leaks on the connections.I let the engine idle for 20 minutes and the needle gradually went up just a fraction below the red line at that span of time.The top and bottom hose became hot and so as the whole surface
of the radiator and coolant on the expansion tank went up a bit.I turned the heater on and blows a very hot air but no smell of coolant.Top hose looks like water is going through when i squeezed it but i didn't felt any of the pulsating or vacuum pressure from the waterpump.

Now here is when I need your assistance and wisdom please.To start with I am not that much mechanically inclined but i want to diy the things before passing it on to the garage.My plan for the next few days will be flushing the radiator coz I'm not sure if the dealer have done it.Change the thermostat,coolant, the expansion tank cap,and check the waterpump if needs changing too.Am I on the right direction?if these bits will not solve the problem i guess i'll take it to the garage for the ignition timing to be checked as the cambelt was also changed when they replaced the thermostat.

Thanks for spending time to read my write up and inputs will be much appreciated.