Random memory issues reported adding items to cart

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.4.8
-- PHP version -- : 5.6.18
-- Browser(s) name and version -- : any
-- Error-message(debug-mod must be tuned on) -- : PHP Fatal error: Allowed memory size of 419430400 bytes exhausted (tried to allocate 130968 bytes) in /home/xxx/public_html/administrator/components/com_hikashop/classes/cart.php on line 310
PHP Fatal error: Allowed memory size of 419430400 bytes exhausted (tried to allocate 65552 bytes) in /home/xxx/public_html/libraries/joomla/filter/input.php on line 980

I had a similar problem that reported memory size of X exhausted and a manual update to that module seemed to move the problem to another location.

It doesn't happen consistently...reports from users attempting to check out say that their browser "freezes" or they get a white screen.

When enabling debugging, and monitoring the website, about 3 or 4 times a day we get the Allowed memory size of X exhausted in ../com_hikashop/classes/car.php on line 310 then immediately followed by another memory error on /libraries/joomla/filter/input.php on line 980

I've tried researching the joomla/filter/input.php as I've seen this before on other sites, but those fixes normally indicated something "upstream" that was triggering the issue.

I have kept increasing the PHP memory limit earlier, and it doesn't seem to matter how much memory I throw at it, so I'm guessing there is a problem with the 3rd party application (JEvents/RSVP Pro) that is adding the item to the cart. I've been having issues with saved carts so I've disabled that feature in Hikashop.

I don't know where else to turn to try and pinpoint whats happening. We can get orders for the same event for hours without any issues, then randomly we'll get the out of memory error. The visitors we've worked with say that it is after they've added the item to the shopping cart, just before the cart is displayed.

Can you offer any suggestions on how to pinpoint this further? I've disabled all of the plugins, then enabled them one at a time, but since the issue can't be replicated by me personally, I really don't know how to track it down.

Thanks