Bit of background, the only plugs I can get locally for my vehicle are copper ones. Because they are a slightly larger size, platinum ones are as common as hen's teeth and the copper ones usually last no longer than 25,000 km (15,600 miles). This is also the manufacturer's recommended interval for copper plugs on this engine. If I want platinum plugs, I need to order them from abroad.
Of late, I've had various problems with my truck - it would accelerate poorly, and sometimes when you floored it, it would continue revving at 4k rpm without much difference in acceleration. Then I had a problem, which I thought was the TPS, followed by another problem which had me check everything from the u-joints to the distributor. The worst was the problem I described as if someone suddenly hit and released the brakes.
Yesterday, I changed my plugs because it's been over 25,000 km since the last change and this is what they look like with 25,353 km (15,845 miles) on them:
The old ones are NGK BPR6E-11 plugs. New ones are OEM NGK BP5ES, even though I was after ACDelco 41-802 platinum, which I couldn't find. Immediately, the engine ran smoother, acceleration vastly improved and the intermittent 'braking' problem completely disappeared. It's a real PITA having to change out the plugs every 25,000 km...considering ordering NGK Iridiums for the next change. 
The old ones are NGK BPR6E-11 plugs. New ones are OEM NGK BP5ES, even though I was after ACDelco 41-802 platinum, which I couldn't find. Immediately, the engine ran smoother, acceleration vastly improved and the intermittent 'braking' problem completely disappeared. It's a real PITA having to change out the plugs every 25,000 km...considering ordering NGK Iridiums for the next change. 
