jQuery ajax callback receiving incorrect json result - json

Question

In my asp.net mvc 4 web app my action in the controller seems to return a invalid data to the jquery ajax callback... The action is doing a ping to an IP and it is called several times from several ajax calls in order to do pings to several IPs.
A piece of my action in the controller (below what is returned):
Response.ContentType = "application/json;charset=utf-8";
Response.StatusCode = (int)(packetsLost < 4 ? HttpStatusCode.OK : HttpStatusCode.NotFound);
Response.TrySkipIisCustomErrors = true;
return new JsonResult()
{
ContentType = "application/json;charset=utf-8",
Data = new
{
sent = 4,
received = 4 - packetsLost,
lost = packetsLost,
percentLost = (int) (packetsLost / 4 * 100),
responseStatus01 = statuses[0],
responseStatus02 = statuses[1],
responseStatus03 = statuses[2],
responseStatus04 = statuses[3],
responseMessage01 = responseStatuses[0],
responseMessage02 = responseStatuses[1],
responseMessage03 = responseStatuses[2],
responseMessage04 = responseStatuses[3],
rtt01 = rtt[0],
rtt02 = rtt[1],
rtt03 = rtt[2],
rtt04 = rtt[3],
ttl01 = ttl[0],
ttl02 = ttl[1],
ttl03 = ttl[2],
ttl04 = ttl[3],
minRtt = roundTripTimes.Count == 0 ? "0" : string.Format("{0}ms", roundTripTimes.Min()),
maxRtt = roundTripTimes.Count == 0 ? "0" : string.Format("{0}ms", roundTripTimes.Max()),
avgRtt = roundTripTimes.Count == 0 ? "0" : string.Format("{0}ms", roundTripTimes.Average())
},
JsonRequestBehavior = JsonRequestBehavior.AllowGet
};
and jquery ajax call:
ajax({
url: '/Controller/Action/',
type: 'POST'
}).then(function (data) {
var _data;
try {
_data = $.parseJSON(data);
}
catch (err) {
alert("Something went bad!!!");
}
}, function (data) {
var _data;
try {
_var = $.parseJSON(data.responseText);
}
catch (err) {
alert("Something went bad!!!");
}
});
1) When action returns http status code OK (200) data.responseText in the jquery ajax callback is empty so I convert data instead (that contains correct data) but for some reason it is crashing when converting using parseJSON(data): error catched says invalid character... I do not know why...
2) When action return http status code NotFound (404) data.responseText sometimes (not always) is empty and statusText within data is "error" so neither I do not understand it....
Action in the controller always is returning the correct data....
Could someone help me to detect this error? or right me in the right direction....
UPDATED:
From ie dev tools, in network, I have observed that some pings are cancelled and then when trying to process them it returns empty responseText to the ajax callback so when trying to parse it, it crashes. I have observed that If I set the timeout for ping to a lower value, for example, 750ms, it is working but if i use the default timeout for ping, that is, 5sec, then it does not work: some pings are cancelled and when trying to serve them they are returning empty responseText that cause it to crash when trying to parse in ajax callback.