node.js web server on heroku - constant memory grow

My app is up and running on heroku on 5 parallel dynos. There is constant load of 500-1500 req/min so up to 25 req/sec. The problem is that RSS memory is constantly growing. For now I am manually restarting app when memory reaches dangerous level (maximum memory used by 1 heroku dyno is 512mb). Memory chart looks like this (upper gray axis is at 512mb limit):

Moments (on chart) when memory is released are when I am restarting app.

The strange thing is that it happens only when there is constant load on server. When there is for example 2 minutes load on server then memory increases and after that in goes down again. So it seems that for some reason garbage collector is not working properly (it is no collecting the garbage until server load is finished and app is not busy).

Is there anything I can do about it? It is not memory leak I think because memory is released when there is no load on a server...

What I tried so far was:

1. Call GC manually in 2 minute intervals
2. Setting --max-old-space-size=300 however nothing happens when memory hits 300mb - it is still increasing.

Maybe there are other options that could help?

Node version is 0.10.20

I used node memwatch package and I managed to gather heap diffs from app:

• http://jsonfiddle.net/auf8d (heap diff between 11 minutes, +37 MB)
• http://jsonfiddle.net/4nvcd (heap diff between 15 minutes, +50 MB)

So there must be kind of leak I guess. The biggest memory changes from first diff:

        ...
        {
          "what": "Array",
          "size_bytes": 9320312,
          "size": "8.89 mb",
          "+": 79086,
          "-": 10215
        },
        ...
        {
          "what": "Closure",
          "size_bytes": 2638224,
          "size": "2.52 mb",
          "+": 36826,
          "-": 184
        },
        {
          "what": "Native",
          "size_bytes": 21471232,
          "size": "20.48 mb",
          "+": 546,
          "-": 0
        },
        {
          "what": "String",
          "size_bytes": 2068264,
          "size": "1.97 mb",
          "+": 36968,
          "-": 1223
        },
        ...

What is Native object (it allocated 20mb mem!)? Could you give me advice on how to investigate what exactly is causing the leak?