O.K. Simple enough though I would have used an external counter instead of an interrupt, then you could get the count without bothering to service the interrupt every cycle.

So, you increment a count every cycle and check the count every second and this obviously is your frequency in hertz.
You said that you disabled T0 and T1 to see if they were causing you problems, what are you using for your one second time base? T2?
Again, are you using assembly?
What type of variable are you incrementing in the int4 routine?

Remember, you need 16 bits for this. The external counter T2 for example would be ideal for this situation. The counter is already 16 bit, and you would have no interrupt latency to deal with.

The frequencies you are talking about, even tripled, do not require PCA.

Allen