Did you not just have tank issues???
Start with the simple stuff, take the plugs out and look at em, they should look the same, check the compression, it should be within 10 pounds on each. Compression checks ok, sigh a sigh of relief and go from there, this is where I start everytime, heck it could be a plug wire not on good. Your mechanic needs to take it to the lake himself, most dont do this but there is no better way to test one than under a load.