Hi,
Running the PIC at 40Mhz and having a prescaler on TMR1 of 1:1 (no prescaler) makes it "tick" at 10MHz. Your sine-table, if I understand correctly contains 72 "steps" so you need 72 interrupts per second per Hz output frequency.

At 120 Hz output frequency you need 120*72= 8640Hz or an interrupt interval of 115.7us. At 10Mhz one "tick" is 0.1us so you need 1116 ticks between interrupts and therefor you should (theoretically) reload the timer with: 65536-1116 = 64420.

At 10Hz you need 720 interrupts per second or an interrupt interval of 1388.9us. That's 13889 ticks so you need to reload the timer with 65536-13889 = 51647.


Now, when the timer overflows and trips the interrupt a lot of things has to happend. The DT-Ints interrupt "engine" has to save all the PBP system varibles and this takes time. By the time the code actually gets to reloading TMR1 with your calculated reload value several hundred cycles have passed. If you then reload it with the "original", calculated value you are effectively "turning back the time" or reclaiming time that has already passed.

What you usually do is stop the timer, make a copy of its content and then ADD the reload value to the value of the copy. Then you put the value back in the timer and restart it. Obviously this too takes a couple of cycles so to be really accurate you need to tweak the reload values to account for it. Basically
Code:
TimerShadow VAR WORD
 
T1CON.0 = 0  'Stop TMR1
TimerShadow.HighByte = TMR1H
TimerShadow.LowByte = TMR1L
TimerShadow = TimerShadow + TimerReload
TMR1H = TimerShadow.HighByte
TMR1L = TimerShadow.LowByte
T1CON.0 = 1  'Restart TMR1
I see that you have 16bit read/write of TMR1 enabled, I don't know how that affects the above but I'd probably turn that off to begin with.

In the INT_List for the low priority interrupt you have the Reset_Flag set to NO for TMR0 - why is that? You don't seem to reset it "manually" in the ISR.

Finally, in your TMR0 interupt, don't do the LCDOUT etc in there. Set a flag and get out of there, then in your main routine you check the flag, if set do whatever and reset the flag.