Paul;
I know you said your attempts at Soundtrack Pro have not worked out for you, but I suggest it's a great program to get familiar with. I've dazzled clients with simple loops, changed as frequently as the tempo of the edited piece suggests. It may take some time to find what you're looking for in Soundtrack, but it's worth the time investment. And the material is free.
I frequently combine samples of similar tempo and key and am often amazed at what I get.
I feel like a composer, and have no musical skills whatever. It's a lot more fun than previewing cuts from a library and I get something that sounds more "custom".
Good luck.
Jay