Can you change the width of the pulses for each coin? This might help you ID the coins.

Also do you have access to the inner parts of the coin acceptor?
There must be a coin detect device that triggers the coin recognition and pulse generating sequence.

Depending on the mechanics can you add a “Coin insert enable” system driven by your PIC?
This will let the user insert a new coin only after the pulses from the device are received so you will create your own gap between coins.

Can you elaborate a little more on this part of your machine (coin acceptor) on both aspects mechanical and electronics?
Does the machine separate and store the coins by type?
Do you have access to any other information from the coin acceptor that can be used in conjunction with the pulses?

Nick