Okay I have no idea if anybody else has had this problem but whenever I hit about 25, 35- 40/45 ish my truck shifts to a lower gear. The problem is that when it shifts it goes from 1500 rpm to 1000 and then just idles and decelerates. I am not putting any more or any less pressure on the pedal. To prove that something was up I put it at cruise control at 35 mph and sure enough it would run at 1500 then drop to 1000 then just back up to 1500 then back to 1000 over and over and over again.

Does anybody else have this problem and or know how to fix this?

 

i had something similar happen to me, have you cleaned out your throttle body and egr valve recently? to me thats the simplest/ cheapest try using throttle body cleaner. there are posts on i believe in the diy section to do both, thats how i learned to do mine. could also be a vacuum leak or could be a faulty IAC (idle air control valve) also. i had the problem when i was idling where it would randomly jump. i was told when your driving and this happens it may be the crank sensor,

do you have any codes? or a check engine light

hope that helps

 

I have a check engine light on. Was told that was because my catalytic converter is bad, but I haven't checked it since. I'll do that later and see what codes I get.

 

To me that sounds like the TCC lockup engaging and disengaging. TCC lockup happens at 40 MPH and rpms would be around 1200. Idle is ~650. You should be able to check lockup by driving at a speed over 40mph, say 50. At this speed transmission should be in OD and the converter should be in lockup. Observe rpms while maintaining a steady speed on the gas peddle and "tap" the brake with left foot, This should release TCC and the rpms will jump up a few hundred rpm as the converter releases. It should then reengage pretty much right away again (foot off brake). Just a thought on something to check...

 

Does the TCC always disengage when I lift my foot off the gas? Whenever I take my foot off the gas and then reapply it it jumps to about 2000 rpms then jumps right back down to about 1600 if I'm going about 55 mph. Is that normal?

 

At 55, driving highway there should be no change in rpm off and on the gas unless there is heavy acceleration making it downshift. When slowing down, at about 40MPH TCC should disengage and may feel like a downshift.
I really cant say what problem you might be having, but if it doesn't feel right try the previous suggestions, keeping in mind TCC lockup conditions. My 2000 has no vacuum to the tranny maybe the 98 does and has a vacuum leak