yarikoptic

After I've done with changing bearings on the crankshaft and changing
upper timing chain in my 1993 Sentra (GA16DE) I got some old problems
back - don't know what I touched or moved that cause that to reveal
again.

First idling is not stable and not always like that. Usually it is ok
at 600-700 rpms if no break is pressed. When I press the break it
starts going up and down from 400 up to 1200 rpms (period of 3-4
seconds). Sometimes the same picture happens even without breaks and
sometimes it works just fine all the time. Is that a vacuum leak
somewhere or something with sensors which deal with idling?

The second problem (old one and I don't remember how I got rid off it
or it just disappeared by itself) is that when I'm going on usual
speed ~40 mph (~2000 rpms) and going from acceleration to breaking by
engine and stay at that equlibrium when engine is not loaded for some
time (0.5 - 1 sec) engine throws car forward and back till I load
engine again.

Thanx for any ideas!