Hello everyone, haven't posted for a long time but am still reading most every day.
While in Yellowstone Park with my RV, my son borrowed my Bonneville. He was on the freeway and the car started to overheat, he pulled off and shut her down. He called me and I had it towed to my home. My very competent mechanic friend went to check it out and found a hard knock and coolant in the oil. I just had the intake gasket replaced about a year ago. At any rate he said he could pull the engine and take a look at the crank and go that route, I suggested that since the car has 155K on it and he was pulling the engine anyway, why not replace it with a used engine. I have another good friend at a salvage yard who found me a 2005 3800 with 42K on it for $700. My other buddy picked it up, did the necessary top end swaps, including the new updated IM gasket. Runs like a champ, but sometimes the throttle feels "mushy", taking about three inches before much response then takes off. I have checked vacuum hoses and top end obvious things. My buddy hasn't looked at it but says he just swapped it over so it shouldn't be an adjustment. I have noticed that it doesn't usually act like this when cold, and it is worse when the a/c is on. Any ideas? I couldn't find any similiar symptoms in my search of the site. Any suggestions would be greatly appreciated, and thanks in advance.

Check you air filter, spark plugs possible, injectors could need a cleaning. Just a few things you can check there