exhaust leak

Help!! My check engine light keeps coming on. I have a 98 1500 2wd with 91000 miles. I went and had the codes checked and it came back with a miss-fire in cylinder 5. Plugs , cap, rotor and wiring are all new. If i reset the light, it stays off during normal city driving. It only comes back on when the car is under a load. This past trip, I had driven approx. 200 miles and began climbing a long steep grade when the light came on , then it started blinking and then went back to steady-on after the climb was completed. It almost sounds like it is running on 7 cylinders. I checked the compression and all cylinders are approx 120psi. I recently just had the intake manifold pulled and a leak fixed in the gasket. While they were at it, I had them install a new water pump, hoses and thermostat. Could anything with these repairs be causing this problem?

I had a friend take a look and he says it sounds like I have an exhaust leak and the O2 sensor is causing these error messages. Is this plausible and if so is there anyway to test for exhaust leaks? Any help would be appreciated since I need to sell the car and my conscious wont just let me reset the light and put the burden on someone else.

Thanks
Bob