Ken yes I would like to see some of your code,
I'm not sure about the HWPM resetting, If I understand correct the pic is monitoring the distance so when it comes close to the object this decreases the HPWM signal ? to reduce spped and turn the wheels at the same time ?
Are you sure that the pic to calaculating the distance fast enough to react with the object ?

I would say that interrupts would be able to act faster than the standard code. it would be nice if you could see how fast the HPWM command is reacting or so fast the pic is processing the data fast enough from the SONAR's