Occasional "Method not found" during jscript eval

[ttomic]

I have asp.net application written in jscript.
On POST request, during Page_Load, I receive string which I evaluate
using jscript eval method. String looks like:
my_fun("arg")
where my_fun is some method in aspx.

body of my_fun looks like:

function my_fun(s:String) : String {
var v:Typ0 = new Typ0();
var x:Typ1 = v.PropTyp1;
var ret:System.String = x.foo();
v.Dispose();
return ret;
}

Where Typ0 and Typ1 are .NET types defined in xxx.dll residing in bin
subfolder.

Once in a day or two (AFAIK in irregular intervals), Page_Load throws
exception:
Method not found: Typ1 Typ0.get_PropTyp1()

Every subsequent call throws same exception.

When that happens, I take following steps:
1. rename bin subfolder into bi_
2. invoke method (now "Runtime Error" exception is thrown)
3. rename bi_ back to bin

And everything is going back to normal.

Asp.net app is running on 2003 Server under IIS 6.
I suspect that it is something related to application pools setting,
but I cannot reproduce bug, just waiting to happen next time.

Can anyone help me?