what is wrong with my front wipers ?

i have a rover 25 [2002] 1400cc. has any one else had this problem ? my wipers judder very intensly after a couple of minuets in the rain & is driving me potty, i have bought several types of blades this year the latest fitted now are bosch, trying to cure it i have had a garage check the linkage system & its ok i clean the windscreen spotless with auto glass cutter. the wiper arm spring tension is tight as normal, is there any thing i have missed that would possibly cure it ? ps. i had a new windscreen fitted about 2 years ago as well.