Vacume problem need help in what to look at

I have searched and did not find the answer.

I have an 89 XJ 4.0, AW4 & 242 and I had vacume leaks that I have gotten fixed but now I have a problem that is hard to replicate in the garage and I don't know what I should look at. What happens is only when driving is I lose vacume pressure after about 1/3 throttle and it does not come back until I let off the throttle. The symptom is they ac vents go back to defrost mode. Some times it only happens a little sometimes it seems like it loses all vac pressure. The vehicle does seem to idle a bit low as well if that is helpful. The problem does happen with the AC on or just the vents. I have replaced the vacume bottle in the bumper and most of the lines under the hood and to the firewall. Its hard to test vacume pressure under the hood while driving. Just trying to decide if there is something I should look at before replacing all the lines in the dash which looks to be a huge pain.

So what should I look at / replace?

Thanks,

Mike

I would check to make sure the vacuum lines are routed right(under hood), it almost sounds as if one of the lines are really old and the vacuum is closing the hose.