Getting Frequent Time out - ios

Question

I am Getting Frequent Timeout errors (not always but some times)although the timeout is set 500 .Chekd the same on postman too Two things one in postman the connection is always keep alive and second one is "The request timed out" coming randomly not always .BUt it just happen as soon as screen opened as i have called the webservice in ViewDidload so it has to at-least wait a bit .before throwing the error
what might be the possible cause for this
Below is my implementation for request
+(void)placeRequest:(NSString *)action parametersString:(NSString*)postDataStr withHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))ourBlock {
NSString *urlString = [NSString stringWithFormat:#"%#%#", IOS_BASE_URL, action];
NSURL *url = [NSURL URLWithString:urlString];
NSDictionary *headers = #{ #"content-type": #"application/x-www-form-urlencoded",#"Connection":#"keep-alive", #"cache-control": #"no-cache"};
NSString *parmetersToPost=[NSString stringWithFormat:#"%#%#",postDataStr,[AppDelegate UserWebCommeParmeters] ];
NSMutableData *postData = [[NSMutableData alloc] initWithData:[parmetersToPost dataUsingEncoding:NSUTF8StringEncoding]];
NSString *logString=[NSString stringWithFormat:#"Here the Url is : \n\n\n\n %# \n\n\n\n\nAction Used is :========> %# \n\n\n\n\n and the Parmeter posted are :=============> %#",IOS_BASE_URL,action,parmetersToPost];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:500.0];
[request setHTTPMethod:#"POST"];
[request setAllHTTPHeaderFields:headers];
[request setHTTPBody:postData];
NSLog(#"All header Sent are %#", request.allHTTPHeaderFields);
[[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:ourBlock] resume];
}
Calling this method as below in viewdidload
[HttpRequest placeRequest:#"check_value" parametersString:postData withHandler:^(NSData *receivedData, NSURLResponse *response, NSError *err){
}];
Error Domain=NSURLErrorDomain Code=-1001 "The request timed out."
UserInfo={NSErrorFailingURLStringKey=http://cms.xa.com/apps/xa/ax/myFuntion,
_kCFStreamErrorCodeKey=-2102, NSErrorFailingURLKey=http://cms.xa.com/apps/xa/ax/myFuntion,
NSLocalizedDescription=The request timed out.,
_kCFStreamErrorDomainKey=4, NSUnderlyingError=0x16587760 {Error Domain=kCFErrorDomainCFNetwork Code=-1001 "(null)"
UserInfo={_kCFStreamErrorDomainKey=4, _kCFStreamErrorCodeKey=-2102}}}