Engine misses at 35-45mph

Are you certain it is an engine misfire? Engine misfires are not typically related to vehicle speed -- consider a possible vibration from your transmission's torque converter as it engages and disengages -- most often right around those speeds. Testing is required.

hi, Does your oil light come on? & Do you notice any type of oil leak from your manifold gasket(s)? because it can be something as simple as oil intrusion/leaking into one of your spark plug well(s) causing the misfire! In this case, you would do a spark plug change and clean out the well or wells that has oil in it with a rag the best that you can, and then obviously change your valve cover gasket(s) to prevent it from ever happening again in the future.. : )

correction: *Valve Cover Gasket(s)..?