Saab: Saab 9000 bucking problem, saab 9000 cd, fuel pressure regulator

Question
Hi,  I just bought a 90 saab 9000 cd turbo last month.  I changed the oil and plugs when I 1st got it.  It ran great for the 1st 2 weeks and then one day while on the highway it just started bucking when the turbo kicked in about 1/2 way up the boost meter.  I took it to a saab mechanic and he told me that I had not used the resister plugs needed for this car.  He replaced the plugs and some other work and I thought that was it.  It ran well for about 1/2 an hour and then started doing the same thing again.  I have since replaced the ignition cassette and wires (all new)thinking that would do it.  It ran well for and hour and then started doing it again.  The really strange thing is that it runs well for 1/2 to 1 hour well and then starts to buck when the turbo kicks in about 1/2 way.  I have no clue what could be doing this.  The only other thing I have noticed is that it seems to start this problem when the gas tank is about 1/2 way down and lower, although, it did this with a full tank yesterday.  Sorry for the long question.  Any ideas at all.  Thanks

Answer
Possibly the overboost switch is cutting out your fuelpump, it may be defective. I would try disconnecting it from the boost line to see if that cures it. Assuming that your 1990 saab has one....otherwise, i would check the fuel pressure regulator, perhaps the APC solenoid or APC control box, once again assuming your saab has them. Resistor plugs should not matter. I use bosch platinums, exclusivly.