89 oldsmbile 98 fixed one problem but still have one

[pinto21632]

3.8 motor 3800. i fixed the backfireing it was a broken spark plug but i still have a problem i drive the car about 10 or 15 miles or if i let it sit in park and run for about 30 to 45 mins then it starts stumbles like a plug missing or more like a surgeing but i put in new plugs.new cam sn.crank sn.new air filter.cam magnet .coil. plus when it is acting up it does not die now .it idels great in park but if i give it a little gas peddle i mean very little just enough to speed the motor just a little more then idel it has what sounds like a miss a low boom boom sound out of the tail pipe it does not do it when it is cool like when i first start. it does it after a 10 or 15 mile test drive when i first start it when it is cool runs like a new car intill it gets good and warm 10-15 miles later if i let it set and cool down it starts and run great then it get warm and it goes bump bump surge .surge its not the maf had that checked i have no codes did th jumper thing . i dont know what to do