Michael, did you solve the problem?
I measured the square wave voltages on IC3 at key down:
Pin3: 5.0 V
Pin4: 3.5 V
I have 2.9 W out at 13.5 Vdc. 20 m. I had to replace C25 and C26, same value (390 pF), NP0.
Do you have ~12 V supply voltage even on key down?
If you want to replace IC3: I found a toothpick very helpful in removing the solder out of the holes.
I use a cheap socket for IC3 without problems.