When climbing a slight hill, it starts to buck in the area of 1500rpms and 43mph but goes away when giving gas to shift to lower gear. It was a minor thing, but it's a bigger annoyance now. I thought of the plugs and coils, so I changed the three front cylinders...problem stayed. Wanting to avoid $400-$500 to change just the rear plugs, I replaced the fuel filter; it seemed to work for a few days, but it's starting again. Also want to avoid $100 to check ignition to find out the rear plugs are okay or face an additional bill, if they're not. Just a note: I had the same problem before and it went away when the transmission was replaced; I'm thinking it was just a coincidence. Any thoughts would be appreciated. I do have 225,000+ miles on the car and 100,000 on the tranny. Thanks.

 

Any check engine light?
Might be a good idea to have computer checked for any codes. Not all problems will illuminate the check engine light especially on first time.
When was the last time the plugs were changed? Condition would fit into problem of "breaking down under load" , usually ign related.