AVR-Chat

I am trying to make a sweeping frequency output using an Mega128. I set 
timer 1 to CTC (mode 4), and set OC1A to output. If I run my program 
normally, the OC1A output toggles nicely at whatever fixed frequency I 
want. But when I start sweeping, as soon as I write a new value to the 
OCR1A register it stops working and the output goes low. If I write 
increasing values (which cause the output to decrease) it works OK, but 
if I write decreasing values it stops. At first I was updating the 
OCR1A registers asynchonously. I thought maybe that was the problem 
(conflicting with TCNT), so I changed it so that I enable the OC 
interrupt and change the compare value there. It makes no difference. 
In the simulator it works OK. Has anybody else had this problem?

Jim