A couple of potential problems you might want to look at:

Code:
freq = 23*RPM + RPM/3		'Calculation of frequency
period = (62500/freq)*16	'Calculation of period
potential for overflow here. While the calculation 'as a whole' might not overflow, intermediate variables may overflow, if RPM is above 2849.

Code:
		pauseus (1000*period)/cnt		'Variable pause
same thing with these lines. If period is over 65, the intermediate variables used in calculating the pauseus value may overflow.