OK,
Your truck will run on 87 and it will carbon up badily, possibly causing some of your problems plus you will loose 1 to 2 MPG on regular, you do need to switch back, it won't cost you that much more especially when you consider the gain in performance.
When was the last time you decarboned the engine, have you changed out your O2's, they are fried at your mileage? Have you changed your cap and rotor, have you verified your new plug wires are not leaking? Are you going thru any coolant?
I would suggest you pull all the plugs on the one side of the engine that is loosing the cat and inspect them for moisture and color.
Chance are you may have the beginning of a head gasket or valve job which will foul a cylinder causing raw fuel or coolant to go thru the cat, fouling it out.
I would suggest you answer each question and if need be I will be glad to talk to you and walk you thru the steps.
Mike
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