I had the same issue. It could be a number of reasons, any of them a somewhat simple to fix.
1. Your starter may be due for a rebuild
2. If your heat wrap didn't work, it may be your solenoid. There is an article out there somewhere that suggests to replace the large spring in the solenoid with a different rate one. I did that one mine and the problem went away.
3. Your timing might be too much advanced, that would cause the same issue, hard to turn over when hot. Back off your timing 2-4 degrees to see if it goes away.