1992 jeep wrangler (4.0l , standard)- power loss after warm up.

First 15 minutes of driving = quick acceleration , good power- normal.
after that jeep is warmed up good, and any acceleration (usually 3rd gear on), going above 2000 rpm - jeep starts to lose power and stumble. if i stay on the gas hard it will pop or backfire. i can't get above 45 mph, just any acceleration above 2ooo rpm and it just dogs out.
replace dist. cap, rotor , wires and plugs. fuel filter is 3 months old - but will replace it next.
any ideas - why it will run good for 15-20 minutes -- then just **** out?
i can turn off and let sit for 5 minutes - and i get normal operation for 10 minutes or so.