Hi mneary,
when I posted part D of the IC isn't connected anywhere I of course meant it was not connected to the circuit, more or less connected as a separate circuit.
I added U1D again and forced the output low with a voltage divider.
As you can see by the simulation result its output is about 490mV. The output pin7 (supposed to have a triangular wave form) stays constantly on VRef (+6V) supplied from U1C.
Also the CPU-load indicates only 4 to 5% whereas with the working circuit without U1D the load varies between 91 and 94% and the oscilloscope operating.
I left the simulation alone for about one hour but there wasn't any change.
I wonder for the reason because I also simulate more complex designs without any malfunction normally.
It looks like U1D is turning off the rest of the circuit.
I would have liked to try any other OpAmp, but ISIS is very limited concerning working ICs.
Regards
Hans
BTW, try to write U1 double dot D. This is what you get: U1