My 1992 Cherokee has been having issues with decelerating. General when I'm driving on a highway (55+ mph) or up steep grades, it will decelerate even though I am placing pressure on the gas pedal. On the highway it will noticeably decelerate for 3 ish seconds, then return to normal (albeit lower speed). 10 seconds (ish) later the same thing will happen. I have to keep accelerating between those periods to maintain speeds. Up paved mountainous roads it is less predictable, and generally decelerates longer. Any ideas on what might be the issue?
For extra reference, I live in a hot environment and have recently had to replace thermostat and had a shop replace the neutral safety switch.

Thanks in advance

 

Is it an Auto or Manual? If it's Auto, is it downshifting gear when struggling or just sticking in 4th? If it's sticking in gear, try adjusting the TV cable that runs down to the gearbox.

Have you checked the condition of your air filter recently to make sure it isn't severely restricted?

My other thought would be a TPS giving bad readings resulting in bad air/fuel ratios but without looking into it I couldn't tell you how to check that out.

 

Automatic.
It's not downshifting, Best way I can describe it is that it loses power/RPMs. Similar to a fuel vapor lock.
I'm not entirely sure what the air filter might have to do wiith it. Could you enlighten me? I'll check on it.

 

Have a go at adjusting the TV cable and see if that helps it downshift when it's struggling:
If this makes no difference try putting it into 3 manually when it's starting to struggle as it will force the box to go into 3rd.

As for the air filter, it's just a quick and simple check but if you drive on dusty trails or it just hasn't been changed in a while, it is a possibility that the engine can't get the amount of air it needs when under the high loads.

As you said it's not downshifting it's most likely not down to the air filter.