Singleton class to display iAds for iPhone

I wrote a Singleton Class to manage iAds. The iAds appear after 5 seconds of user inactivity. The idleTimerExceeded call generates a notification to show the iAd. This code is great for my requirements, but since I'm new to iOS development, my application sometimes unexpectedly freezes after integrating this code. This code leads to many warnings, etc. I would like to optimize my code in terms of memory and performance.

I would be very grateful for your kind suggestions and feedback.

Below is my code:

iAdSingleton.h

#import <Foundation/Foundation.h>
#import "AppDelegate.h"
#import "iAd/iAd.h"

@interface iAdSingleton : UIViewController<ADBannerViewDelegate> {
    ADBannerView *adView;
    UIViewController *displayVC;
    NSTimer *idleTimer;
    BOOL isItFirstTime;
}
@property (nonatomic, retain) ADBannerView *adView;
@property (nonatomic, retain) UIViewController *displayVC;
@property (nonatomic) BOOL isItFirstTime;

+ (id) shareAdSingleton;
- (void) resetIdleTimer;
- (void) idleTimerExceeded;

@end

iAdSingleton.m

#import "iAdSingleton.h"

@implementation iAdSingleton

static iAdSingleton* _sharedAdSingleton = nil;

BOOL bannerVisible = NO;
BOOL controlAccessBannerVisibility = NO;
@synthesize adView, displayVC;
@synthesize isItFirstTime;

#define kMaxIdleTimeSeconds 5.0

+(id)sharedAdSingleton
{
    @synchronized(self)
    {
        if(!_sharedAdSingleton)
            _sharedAdSingleton = [[self alloc] init];
        return _sharedAdSingleton;
    }
    return nil;
}

+(id)alloc
{
    @synchronized([iAdSingleton class])
    {
        NSAssert(_sharedAdSingleton == nil, @"Attempted to allocate a second instance of a singleton.");
        _sharedAdSingleton = [super alloc];
        return _sharedAdSingleton;
    }

    return nil;
}

-(id)init
{
    self = [super init];
    if (self != nil) {

    /*                  Initialize The Parameters Over Here                  */

        //adView = [[ADBannerView alloc] initWithFrame:CGRectMake(0, 480, 0, 0)];
        adView = [[ADBannerView alloc] init];
        adView.currentContentSizeIdentifier = ADBannerContentSizeIdentifierPortrait;
        self.adView.delegate=self;
        [self resetIdleTimer];
    }
    return self;
}

-(void)dealloc
{
    displayVC = nil;
    if (adView) {
        [adView removeFromSuperview]; //Remove ad view from superview
        [adView setDelegate:nil];
        adView = nil;
    }
    [super dealloc];
}

-(UIViewController *)viewControllerForPresentingModalView
{
    return displayVC;
}

- (void)bannerViewDidLoadAd:(ADBannerView *)banner 
{
    banner.hidden = NO;
    if(!bannerVisible){
        NSLog(@"Banner Changes 1 - Purpose: Visibility");
        // [UIView beginAnimations:@"bannerAppear" context:NULL];
        // banner.frame = CGRectOffset(banner.frame, 0, -100);
        // [UIView commitAnimations];
        bannerVisible = YES;
        controlAccessBannerVisibility = YES;

    }
}

- (void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error
{
    //NSLog(@"Unable to receive Ad.");
    NSLog(@"Banner Changes 2 - Purpose: Unable to Receive Ad.");
    banner.hidden = YES;
    if(bannerVisible){
        [UIView beginAnimations:@"bannerDisappear" context:NULL];
        banner.frame = CGRectOffset(banner.frame, 0, 100);
        [UIView commitAnimations];
        bannerVisible = NO;
    }
}

- (BOOL) bannerViewActionShouldBegin:(ADBannerView *)banner willLeaveApplication:(BOOL)willLeave
{
    NSLog(@"Pause anything necessary");
    return YES;
}

- (void) bannerViewActionDidFinish:(ADBannerView *)banner
{
    NSLog(@"We now resume to normal operations");
}

- (void)resetIdleTimer {

    if (!idleTimer) {
        idleTimer = [[NSTimer scheduledTimerWithTimeInterval:kMaxIdleTimeSeconds
                                                      target:self
                                                    selector:@selector(idleTimerExceeded)
                                                    userInfo:nil
                                                     repeats:NO] retain];
    }
    else {
        if (fabs([idleTimer.fireDate timeIntervalSinceNow]) < kMaxIdleTimeSeconds-1.0) {
            [idleTimer setFireDate:[NSDate dateWithTimeIntervalSinceNow:kMaxIdleTimeSeconds]];
            /*
             Notification: HideAd
             */

            NSLog(@"Notification Generated For HideAd");
            [[NSNotificationCenter defaultCenter] postNotificationName:@"HideAdBanner" object:nil userInfo:nil];

        }
    }
}

- (void)idleTimerExceeded {

    AppDelegate *appDel = (AppDelegate *)[[UIApplication sharedApplication] delegate];

    if (appDel.adVisible == NO) {
        NSLog(@"Notification Generated For ShowAd");

        /*
         Notification: ShowAd
         */

        if (controlAccessBannerVisibility == YES) {
            [[NSNotificationCenter defaultCenter] postNotificationName:@"ShowAdBanner" object:nil userInfo:nil];
        }
    }
}

@end