engine problems on freeway

I just installed a remanufactured 4.0 liter engine in my 1992 XJ. I replaced both temp sensors, the oil preesure sending unit, and installed a new distributor, coil and crank position sensor. The only sensors I didn't replace are the two on the throttle body. After breaking in the engine for 500 miles on surface streets during which the engine ran great, I decided to take it up on the freeway. At speeds around 70 mph the engine starts to cut out. At first I thought the transmission was downshifting to a lower gear, it is pretty violent, but the next time it happened I noticed the rpm on the tachometer was dropping. If it was the trans downshifitng, the rpm would go up. I repeated this several times and always the same, a pretty violent shaking and a drop in rpm and the engine recovers only to repeat again. This only happens on the freeay around 70 mph. On surface streets the engine runs great. Could this be something related to fuel as in the throttle boby or throttle position sensor etc. Any suggestions would be helpful. Thanks.

On surface streets, push it hard and run the rpm's up before each shift. Does it do fine?

I've taken the jeep out on a secluded section of road and ran it full out. Good hard shift points through all gears no problem. Get it up on the freeway and cruise for a couple of minutes at 70 mph, slightly let off of the gas and reapply the gas and engine cuts out. The jeep will shudder and feel like it is backfiring but it's not

If the engine is so sensitive to slight changes in throttle position that it bucks and hesitates, it could be the TPS that's at fault. The check engine light should come on if that's the case but even if it doesn't, you should be able to retrieve any fault codes that might be stored in the engine computer.