I have a 99 mustang v6 automatic 218500 miles with a fairly violent amount of vibration from 1000-2000 rpms that nobody can make sense of. it idles smooth, crawls forward smooth, but upon hitting 1000 rpms it shakes horribly, then smooths out again at around 2000 rpms or 60mph. After that, the higher rate the better. it does it in gear, park or nuetral. does it with the serpentine belt removed so not accessories. transmission shifts beautifully, no jerking or slipping, all mounts are freshly changed. Throttle and maf cleaned, fresh filter, clean fluids, new spark plugs. it also has this same awful vibration on a cold start then goes smooth after ten seconds or so. its a weird one.
Well start by looking for vacuum leaks and pressure test the fuel pump to see if the fuel pump is weak. Past that it could be a whole bunch of things like worn out. Fuel injectors. Ignition system failed here but start with the simple things first
