Well, if your workflow constantly fails to do something it will reach the CPU execution time limit a
Well, if your workflow constantly fails to do something it will reach the CPU execution time limit at some point 
Compute time per Workflow | 30 seconds of active CPU time

An RPC stub was not disposed properly. You must call dispose() on all stubs in order to let the other side know that you are no longer using them. You cannot rely on the garbage collector for this because it may take arbitrarily long before actually collecting unreachable objects. As a shortcut, calling dispose() on the result of an RPC call disposes all stubs within it.