It's hard to say without feeling it myself,but i'll take a stab at it. As a driveability tech most ignition misfires that I get are either from a dead stop or at the 40-highway speed accell. and these get misdiagnosed most frequently as transmission problems. The best thing to do and the first place to start is getting it scanned for codes then go from there that way you know which direction to take as far as diagnosis is concerned, ignition or trans etc.
I agree, at that speed the transmission is in it's highest gear and the engine is under it's highest load. A misfire is most likely to occur under these conditions and be more pronounced than in a lower gear and lower engine load. As suggested have it scan tested to confirm this. Mechanic may be able to simply test drive to determine where the problem area is. Does the check engine lamp flash when this happens? Is the check engine lamp on any time the engine is running?