check engine only when towing

[clintryplug]

I have a '94 Isuzu P'up (4 cylinder) that the check engine light comes on only when I'm pulling my small 14' fully rigged aluminum Jon boat (700lbs gross load with trailer). More specifically it really only does it when I'm on the freeway (up around 60-70mph) and even more specifically when I press the accelerator. As long as I have the accelerator pushed (at these speeds) it stays on but the second I take my foot off of the accelerator the light goes off then again comes back on when I depress it again. I've been pulling this load for a couple of years and have not had this happen until recently, I know a 4 cylinder p'up is not your ideal towing vehicle but I would think that 700lbs shouldn't be too much for occasional towing. The light doesn't come on at all when I'm not pulling the boat. Any ideas?