Very Hot water coming out of overflow pipe

Question

6 year old Heatline boiler. water out of overflow and backing up to boiler.Pressure guage up and down.Hot water in house only lukewarm.
Heating on but not very hot, and when on pilot goes on and off.
Air pockets in radiators, but when bled, only water, not air comes out.

Have been told it could be thermistor or a stat problem.Any ideas?

Siobhan.