Bucking and wacky speedo

96 auto 4.0 with 242 390K on the odo.

Recently the speedo has gone wacky - after about 35 mph it starts bouncing anywhere from 40 to 125.

Also has started sometimes "bucking" when accelerating usually somewhere around 50 mph. does not go away unless I let off the gas and reapply.

Also CC no work - I am guessing because of speedo problem.

Also getting a 138 code for O2 heater. I do not have a rear O2 sensor connected - my cheapo generic replacement cat did not have a bung.

I changed the O2 front sensor a few weeks ago - but the speedo issue started before that.

Wondering if some of these problems sound related? or just coincidental that they seemed to start at the same time.

Another question - I have read that the heater circuit for front and rear sensors are connected. If I have no rear sensor after the cat will this affect the operation of my front sensor? and which fuse is the one to check. I have no listing for O2 in my pdc or fuse panel diagrams. Does the power just come from the ASD relay?

Traced bad wiring above the transmission - fixed speedo and Cruise this weekend.

Did not have any bucking today, but did not give it a good run.

Just wondering about the O2 heater circuit question. I am guessing I will always have a PO138 with no rear sensor, but just want to know if this will negatively affect operation of the actually important front sensor.