engine misses occasionaly at 2600rpm

i'ved searched and have not found quite this problem. whan i'm crusing along at around 70-75mph i tach out at close to 2600rpm. it seems that mostly at this value i start getting rpm drops of around 200rpm. it stutters, vehicle bucks at bit and rpm comes back up. at first, if i was reved above that or below it it ran ok. idle is good. i put in a new tps today but same problem occured. at one point i put the tranny (aw4) into third and was running it at around 3400 rpm. this worked well for awhile. but it it too started to stutter the same way eventualy. and it almost seemed that if i got close to the 75mph in this gear that's when it started. up untill that , i would have said rpm but this was too coincidental. does the speed sensor affect any systems that could cause this or is it just a dumb coincidence? i'm running a bored out tb on a 2000 manifold with a banks header on a 91' and it had been running fine up untill about a week ago.
any ideas as to where else i should look?

Did the check engine light come on? Any fault codes?

no check engine or fault codes. though when i pulled the casp my ciol wire was carboned up pretty bad and the rotor was all wanked out. put in anew cap /rotor.mopar wires and plugs and on a short test run it was fine. i had replaced that stuff with ion the last year so i really wasn't expecting it to be messed up already. i'll post up how it runs on a long freeway ride tomorrow.

just keeping things up to date. this thread should now be considered closed. it looks as though the simple tune up took care of the hesitation. it ran fine at freeway speeds monday and has it's power back too. to bad it always seems that the simplest items get overlooked when trying to find problems.
thanks again to the naxja members for their help