Has to be breach of contract as I will be fairly sure they agreed to pay you the correct amount.
It may or may not have reasons to which you can break but not paying you correctly will be one anyway* – the contract, for obvious reasons, cannot trump actual law. Also bad publicity for them if they do fight.
Might want to cite as many material breaches as you can as well.
Personally i would write to them explain your reasons say you want to terminate without any penalty and of they object you will go to court and generate lots and lots of publicity and hope they just let you without complaining.
* IMHO and IANAL but being paid correctly and on time is an implicit contract term anyway even if it is not explicitly stated in the contract. I will be surprised if it is not stated.