System cycles properly, Blower runs 90 seconds, gas valve opens, ignitor glows, gas ignites, nice Blue color, shuts down in 3-4 seconds, code 34 blinks on " ignition proving failure, furnace relights, does same thing and goes into Lockout. Tried re-setting Temp rollout safety switch. Could it be oxide buildup on flame sensor??

Controller circuit board looks okay. 3 amp fuse is good. Should I call for service or can I clean the the flame sensor myself?

It not only could be, but in all probablilitiy is, oxide on the flame sensor. Remove the flame sensor & clean well with a Scotch Brite pad or very fine steel wool. Do not use any kind of sand paper. Re-install, try it, & let us know what happens.