I have the problem that the calculation time can be 100 to 200ms for the same code on different runs.
So sometimes the puzzle gets solved and sometimes not. The code is deterministic. So what is the calculation time relaying on?
It’s probably the garbage collector. After all, he is a bit lazy and only works if he feels like it. 
But your numbers sound a bit off. How did you measure the time?
I had an unsolved puzzle (time out) sundenly register as “solved”, although i just misclicked the submit button, in order to test an idea on older code, and had just pasted … Older code that was previously rejected was accepted.
(I think this was in scala, so it could be the garbage collector, but still the run time appeared to be undeterministic)
I use Python and I experimented a little bit with the garbage collector but this didnt seem to change the execution time.
Basically, the servers that run your solutions are not strictly identical and they are also shared, which may influence a little the results too.