PMT calculates the payment for a loan but assumes a constant number of
payment for the loan (constant installments). My requirement is as follows:
a. Interest varies between months (could be the same for a long time and
change again)
b. The monthly installment is the same irrespective of the interest change.
c. The loan tenure changes because of increase/decrease in the interest rate.

I have the same issue with IPMT and PPMT as well.