You are probably experiencing a proper condition called decel cut-off. The conditiona are something like this: the idle switch is closed and the idle is above some amount (the amount depends upon the temp).

It is probably about 1500rpms on a hot 4.5 (was less on 3.5's and they always experience this phenomemon. When these conditions exist the fuel is shut off because the car should be driving down the road with a decel in mind to have those conditions. After the fuel is cut it is restored when the idle drops below say 1200 thus the up and down in an exact manner.

You will need to get the rpms down. I like timing and since timing will make sure a 3.5 decel cuts off at idle when cold, I have done a run around on a couple of them. One can disassemble the throttle switch and disconnect (break) the idle switch contact. After this the throttle will never be seen as closed and never will decel happen.

Or..... you find out why your idle is too high at that timing. 4.5s don't have this problem unless way out of spec. I would look at the auxliary air valve if you can't turn the idle down. Either that or an air leak somewhere.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician