Edited Unassigned: Parse exception but only sometimes [100]

Hi I have simple code,that 1 for 10 request edns with this error, any gues why ? multithreads or sth else?

it blows up in the first line

> engine.Evaluate("library(foreign)"


I have now more information about my problem,
I have simple web application to generate some report, so if I run this as single web page , one by one, it works like 8/10 times, but if I open this site in 3 tabs, and click them in the same time to generate report, then it fails,

I tried to make my report method thread safe this using
> Monitor.Enter(syncRoot);

So now I download source code to see where the exception fails and this is that part:
> if (!vector.First().TryEvaluate(GlobalEnvironment, out result))
Line 458: {
Line 459: throw new ParseException();
Line 460: }
Line 461: return result;

in deep I saw sth like this: Error: C stack usage is too close to the limit
Any help what I can do ?