this is a cheap fix that works probably 90% of the time. Get some
"sea-foam" spray deep penetrating lubricant and some intake cleaner.
Working one line at a time, take off the vacuum hoses that you can
find, spray with the intake cleaner until it appears clean inside. Then
repeat right at the intake, remove the hose, make sure the car is off,
and thoroghly douche it out. Take seafoam and saturate the inside of
the intake as well, careful to blast into any nooks and crannies.
Replace intake hose. Take off vacuum line that feeds into the valve
cover. Spray seafoam liberally into the head.
Start car, (it will be rough for a min till all the intake cleaner is
burned out). Then, once the car is running, remove the vacuum line that
feeds into the intake, spray seafoam into this line, taking care not to
stall the car, little bit at a time. I would advise you empty the can
into the system, it will start to "smoke" out of the exhaust, fear not,
this is the excess carbon and sludge in your motor. As soon as your can
is empty, turn the car off, allow it to sit for about 10 minutes, no
longer than 15. (the seafoam will be disolving deposits in the motor
but you dont want to risk any damage by letting it sit too long.) Then
fire it up and go for a joy ride, the car will be smoking like a
bastard. Drive it at high RPMs (3500-5500) until all smoke is gone.
(the high RPMs help to burn the deposits and remove all the crap). I do
this about once every 6 months with the bimmers and Rovers to keep the
motors in good shape, if this doenst fix the problem I would consider
cleaning out your cold start valve(does that 3.5 have a CSV?) or
stepper motor.(electical cleaner)

I wouldnt say seafoam is for tired old motors by any means, granted,
its not the most well known product yet, it is highly regarded by many
auto groups, Land Rover, Mercedes AMG, Porsche and it was recommended
to me by a mechanic here in SLC who has been doing work for the local
chapter of the BMWCCA here for years. More to the point, I would say to
use seafoam if the motor is in good condition only, a motor that has
bad seals etc.. will see a definate degradation of performace as all
the crap in the motor will be removed and if you motor is relying on
sludge and build up for sealant it will perform more poorly. Seafoam
will more restore the motor to like new spec, its a little more
aggressive than other products like BG44k, but it does work alot
better.
Sorry if I sound like a commercial but I live and die by this stuff,
and if your motor is pretty tight I would bet you will be amazed at the
difference in power after a good "seafoaming".
Ive run it in Series 1 Discoveries, 04 Freelanders, e30 M3s, e36 M3s
and 318, 88 and 91 E32 735s, and a few e28 535s. Only one that ever
gave me problems after the seafoaming was a 535 with 410,000 miles on
the clock still running the orig. motor, it cleaned it a little too
well and I ended up doing a motor swap as it was cheaper than a rebuild.