This is probably due to the train moving along the tracks to its destination and therefore your position is constantly moving. Adding to this is the train maybe go through an area where signal is weak this is usually known as the countryside. Yes it is a known issue for the above reason.
3G has always been deemed bad on trains. It can be a matter of what Train Operator is concerned, the train's construction materials and the area of the UK involved.
I think it is discusting that the people who can afford to pay for WiFi on trains (First class payers..not persons) get it for free, bnut other folk have to pay.