Call GC.Collect In Custom Thread Pool?

I have created thread pool which have 5 threads. Both of them will start when my application start up. The problem is I created so many objects in thread loop call back function on each threads, and the memory is increase by second when app running, this is thread loop call back function:

void ThreadLoop(){
while(true){
var checkItems = _workItems.Where(w=>w.ActivedTime > 3).ToList();
foreach(var i in checkItems){
_workItems[i.Id].ActivedTime = 0;
_workItems[i.Id].ExecutePostBack();
}
//Recreate new _workItems dictionary
_workItems = _workItems.Where(w=>w.ActivedTime > 0).ToDictionary();
// Sleep thread to free up momory
Thread.Sleep(1000);
// Call Grab collector free memory
GC.Collect();
}
}

I'm confusing that, is calling GC.Collect() on each threads like that good or bad ? Dose it cause bad performence ?