You are encountering the dreaded "last-minute-bump". This tactic often is joined by the "they-can't-bitch-too-much...if-they really-want-this-house" and is a staple for the less scrupulous of the mortgage brokers out there.
The advise given so far is the best, just walk. My further advice would be to look at all of your options andthen go with a rival mortgage company.
As a further note, your interest rate needs to be locked with the lender at least a day before your closing....maybe longer. If they are now saying that you are going to have to pay 1/2 a point to get the original quoted rate, then this is what probably happened:
They never locked your loan and now they cannot get you the rate that they quoted. To shift the blame from them and on to you, they went over your credit report (BTW, the SAME credit report that they used to get you the loan in the first place) with a fine toothed comb in order to find ANYTHING that could cause a bump in the rate. This is a problem because there is NO SITUATION (in a standard Conventional conforming loan) that would cause a increase in the rate. The ranges for these loans is usually 3 pts from cheapest to most expensive.....and the lender does not care where in that range the loan is locked. In addition, creidt issues in the conforming loan are addressed way before the rate lock discussion. Basically, if you are approved for a conforming loan, then you get the rate range from the lender, it is up to your broker to set and lock the rate. So, if you have a problem with the rate, that problem is 1000% with your broker....I'll say again, walk.