Hi, question, have you got the gnd of the CRO (sorry oscilloscope) connected to the gnd/neg of the arduino?
A picture of your connections and CAD or picture of a hand drawn circuit in jpg, png or pdf will help.
The problem here is that you have all the information, circuit, CRO display, sketch etc and we have nothing.
So to keep the number of posts to a minimum to help with your problem, we need all the above data so we can catch up.
Hope to help.
Tom..... 
PS how do you get 10V p-p on a pic micro running at 5V supply?
PPS. Don't use measure function, use the screen grid and do it by hand, the measure function could be measuring glitches. Or if using x10 probe is it compensated.