No doubt you gents know more than I do; I had read that a root problem with it acting this way was the flame sensor listed as the the next thing to look at on a couple of sites (I put in new rollout and hi-limit switches, new blower/motor). The sensor has never been out having this unit 20 years. When I gently tap on the unit with the back of my screwdriver handle near the hi-limit switch it works fine for a while.
I will try touching the wire though next - could be my tapping trick is jiggling that wire just enough. Pressure checks or a new valve is something I'll let a tech do if the sensor doesn't do it....thanks!