Interrupt 1 (digital 3) slow on Mega 2560?

Has anyone else had this problem? I've got an interrupt routine attached to int 1 (pin digital 3) and I'm running micros() in loop. Here's what I've got:

attachInterrupt(1, introutine1, FALLING);

When loop is running I see no delay in printing micros() out to Serial... if I pull the jumper cable from the interrupt it takes almost 5 seconds (sometimes as little as 3) to trigger the interrupt. Same thing if I use change or low. Again, the printing of micros() is not delayed and their doesn't appear to be any pausing of loop.

Shouldn't this change be registered immediately? I really need an oscilloscope...

Any ideas? This is really bugging me!

Perhaps if you posted your code?

Don't just pull the wire from the pin, (i am assuming you have the pin wired to +5V) connect it to ground. Never assume a pin is going to go to a specific voltage if you aren't taking it there yourself. When I am using interrupts I tie the Int0 pin to +5V through a 1K resistor and then pull it low by tieing it to ground.

pugglewuggle: ... if I pull the jumper cable from the interrupt ...

Interrrupts don't have jumper cables. Perhaps if you describe in more detail how this is wired up? What wire is going where? As kf2qd said, if you simply leave the wire floating "in thin air" then it could have any value on the input pin.