Over flow tank

Hi guys,

I have been running my 2005 TE 250 (now 310) for five years now and never had any problem. I changed bigend and rod togethet with new piston and cilinder 1 year and a half from now and never had any heating problems, not even on steep sandy trial climbs (1st and 2nd gear stuff and clutch abuse). However since a couple of weeks I am having some heating problems. The boiling coolant gets in the overflow tank and should flow back but it doesn't, so it overfills and then some of the coolant gets out of the fillercap. I did notice that some coolant was pressed out the overflow cap.
I found out that the bike was running hot last week because I suddenly lost all pressure in the clutch and found out that the hydraulic fluid in the clutch pump was boiling. Looking into the radiator I see a regular flow of liquid, so it does seem to be a waterpump issue. What coud prevent the coolant to flow back into the radiator aftercooling off? Why dores it run hotter now? No dirt in coolant and no smoke from exhaust so I don't think it will be the headgasket either?

Any ideas?