Take a look at the original sentence again:
Slouched in his seat, Michael tried to pay attention to the grammar
lecture the only problem was that NBA statistics
and images of pretty women in tight miniskirts were more interesting than
subordinate clauses and participle phrases.
You chose to fix it the correct way:
Slouched in his seat, Michael tried to pay attention to the grammar
lecture. The only problem was that NBA statistics
and images of pretty women in tight miniskirts were more interesting than
subordinate clauses and participle phrases.
Michael tried to pay attention to the grammar lecture is a main clause. The only problem was is the second main clause. You are always correct when you use a period and a capital letter between the two main clauses that create a fused sentence.