Strange problem
 

Ok so I have a 97 dodge 1500 with 138,000 miles. Just put on magnaflow hi flow cat on it but as of now that is all for the exhaust( rest is in mail). It had no fan so we put a electric fan. And put a new thermostat in at 195 degrees. When I first start it up and it's cold it's peppy and responsive,but when it's fully warmed up at 195 I can hit the throttle and the rpm's act normal when they go up,but when the rpm's come down after hitting the throttle it goes well below 1000 rpm's and what's to die but will come back up to proper idle at about 8 or 900 rpm's,s. keep in mind it has no o2 sensors yet. What do you think could be the problem???

Probably due to not having any o2 sensors to go off of when it goes into closed loop mode.

+1... Finish the exhaust first. And install NTK or denso brand o2's. Then see what the idle is. Should be around 650. And disconnect the negative battery cable before installing the new o2's. This will reset the pcm, so it can learn the new values/parts.