Is there an alternative in JavaScript of getting time in milliseconds using the date object, or at least a way to reuse that object, without having to instantiate a new object e
As far that I know you only can get time with Date.
Date.now is the solution but is not available everywhere : https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Date/now.
var currentTime = +new Date();
This gives you the current time in milliseconds.
For your jumps. If you compute interpolations correctly according to the delta frame time and you don't have some rounding number error, I bet for the garbage collector (GC).
If there is a lot of created temporary object in your loop, garbage collection has to lock the thread to make some cleanup and memory re-organization.
With Chrome you can see how much time the GC is spending in the Timeline panel.
EDIT: Since my answer, Date.now() should be considered as the best option as it is supported everywhere and on IE >= 9.