If you continually start in a really hard gear you are really mashing several parts on your bike. Try to do a start in an easier gear. It could be slipping because
a) Chain is stretched
b) Cassette is worn in the gear you use most
c) The freewheel is 'too' whobbly and when you really mash down it turns in just enough to force a skip
All 3 are feasible and without being there to watch and listen myself I can't offer much more.
It could also be a loose crank. I used to get a loud cracking noice. It was where the crank arm engages with the spline. I have also had a loose bottom bracket which gave a similar sound.
