Volvo Repair: 93 940 shuts of while driving on open highway, thr radio, 1993 volvo 940

Question
hi Roger
i have a 1993 volvo 940 which has 170K on it. i just pick it up a few days ago. was inspected on lift and road test fine. from privie owner through small dealer. on the way home it shut down while at 55 mph and then back fired and started running again. it appeared that this happened when i was reaching for thr radio or wipers. i thought that i may have hit the ign key. here my obversions. no warning lights came on even though this took over 4 sec between shut down and the restart. the tach dropped to zero. the car was still in drive (auto trany), these things i know happened. i also beleive that the temp, fuel & speedo shut down for those few sec. this happened again this morning at 60 mph and this time i moved the key back and forth and it again restarted and ran fine. i have owned volvos for 28 years and have a lot of experince working on them. i think that the ign switch is the problem. the test that i preformed was to very slowly turn the key back until the car shut off. all the items i listed shut down & the warning lights didnt come on until i switched the key back towarded the full on position. before i moved it back toward the on position i observed that these items still fuctioned, heater fan, radio, wipers & turn signal. i tried this again(turing key back) while driving in gear at 35 mph and got the same results (items stayed on). im thinking that there are 2 power legs off the ign switch elec plug, one which controls power to the ign and the other for the other items. having just getting this car i dont have a shop manual for it yet. my 740 book goes to 88 or 90. do you have any thoughts on this problem or have you come across it before. i feel that i am in the correct direction and found this link and thought you might be able to add to it. i dont feel this is a fuel or electronic ign problem, i think it is more of a electrial   curcuit problem. also, can you recommend a got service or shop manual. sorry this is so long but i wanted to give full details.
thanks Broderick  

Answer
Broderick, It is  a good diagnosis on the ign switch but I want to go more with a secondary igntion problem like the speed sensor or power stage.  The only books that are worth a damn are the factory manuals but they can get expensive.  Try a chiltons or haynes book.  They help but are bad.  Roger