Dewan Housing Finance - Frequent interest rate changes

My DHFL loan code is ********. They are increasing the interest rate frequently, at present they have increased it up to **.** which is not there in any of the banks. Last year itself they have changed interest rate around four times, if we contact them they say as we did as per RBI rules, its not in our hands they say moreover they threaten that the document is duly signed by you and we are entitled to change the interest rate whenever we want, agreed they can change interest rate whenever they want but this much frequent? and up to **.**? I have paid *.* lacs and *.* lacs as part payment. What's the use of part payments if they keep increasing the interest rate as and when they wish? who changes ROI * times in a year? If months keep increasing, what's the use of part payments? Please help.