recently my car has been acting up, let me start by saying i have been a mechanic for 8 years and am good at diagnosing and repairing cars......but this is a different story as u will read......... I have been driving a 2005 mustang gt with 35000mi....accessories are CIA (says it does not require a tune) wheel spacers, full boy racer body kit w/ duck tail instead of the wing, modified mufflers (cut them open and hollowed them out).....was chipped until i was in a fender bender.....not bad, no airbags, no dammage to engine at all i drove away and car was fine for months.....i recently put chip back in to see if that was the problem but no difference. oh by the way it is a chip not a tune!!! best of all i took the manifold runners out and drilled the rivits that hold them to the shaft. there is one per cylinder and they are designed to stay closed until 2750 rpm then they go wide open. thats why the 05-09 mustangs would lag until u hit 2800-2900rpm then it really pulls. taking them out was the biggest power gain so far. more noticeable than the chip and cia!!!!!!! no engine light and passes Ca smog test no problem.....if you do this you will love the results! write me if u want to know how. ok, now for the issue at hand. my car has gradually been running rougher and rougher. idles fine and accelerates fine. drives good when its could and drives good warm! it will only occur one time a day at the same distance and the same driving conditions. same temperature same speed. no engine light and no other issues. i will be driving from home, about a quarter mile to the freeway on ramp and cruise at 70 mph, give or take a few,. i will make it about a 2.5 miles and it will buck and stumble. it will buck and stumble maybe 4 or 5 times within 1 mile and or about 1 minutes of driving then it goes away!.....I have a genisys evo 4.0 and can view live data, codes, mode six, component tests, pending, misfire data, sensor data and repair info and i cannot find anything too far out of value, nothing has tested bad and nothing has changed for the last year..........interest yet. the problem i just described happened 3 days in a row....all the exact same. now for the last 2 days i notice it runs rough at 1600 to 2500rpm and only in neutral but fine under load and fine on hard acceleration and idle. still no codes or info saying anything is wrong and sensors read good, all that seem important and that i can monitor.....and there are a lot! does anyone have any ideas, i dont even know where to start. checked throttle body; clean, checked oil, full and clean; added 8 gallons of shell premium to almost empty tank (was running 87); no difference. new air filter; same! disconnected battery for(i know, overkill) 7 hrs; no change. i do not see a problem but i can feel it! im going to break out the scope tomorrow does anyone know how to approch this? anyone have a similar problem? does anyone have a discription of tsb's? wiring diagrams? HELP!!!!!!!!