Hot water keeps tripping between hot and cold?

I have an 6 year old Baxi combi, Platinum 40 HE and when you call for hot water it cycles hot then cold. Burners yellow light comes on for about 20s and the temperature on the boilers gauge rises to 89 degrees then the yellow flame goes off for 20s etc. The water has been getting progressively cooler over the past few weeks. Have replaced the temperature sensor in a bid to repair but to no avail. The central heating side is fine.