In any case two seconds of cranking is a satisfactory

time for an engine to start. Not sure if you intended this

to be taken literally. Also in the subject you say that

sometimes it is slow and others it is fast. If this refers

to the actual cranking speed, Humberto may be right

in pointing you to voltage and starter brushes. Another

area to check is the connections on the big cables from

the battery, both the hot one and the ground one.

Because of the large currents these carry, any small

resistance can make a big difference in cranking

amperage and thus starter torque and speed.

Another link in this chain is the starter solenoid.

In spite of the fact that I have 3 Volvos all over 26

years old I have never bought a new starter or

solenoid. I do have a couple extra starters that

I have re-brushed and cleaned the contacts on

the solenoids several times each. Once you get

the solenoid apart it is easy to clean the contacts

and they can be built back up using silver braze

alloy ("silver solder") by a competent torch operator

at little or no cost. The key to this is getting the potting

off the plastic cap of the solenoid so you can take the

two screws out that hold the cap on, unsoldering the

wires to the solenoid coil that are in hollow rivets in

the cap, then getting the funny washers off the contacts

(which are like 8mm bolts with copper heads) so you

can get them out of the plastic cap to work on them.

They will usually have black, shallow pits on them that

can be easily filed out, being CAREFUL to keep them flat.

Same is true of the moving contact. After about 3 iterations,

you'll have to build them up with silver braze alloy but that

will probably be several years off. After you get the contacts

clean, put them back in the way they came apart, guide the

wires into the hollow rivets (remembering which one had

the double wire), re-solder the wires and reinstall the cap

with the two screws. Then put the funny washers and nuts

back on the contacts and put the solenoid back on the starter,

being sure the nuts holding the battery cable and the lead

to the starter motor are good and TIGHT (and clean!)

This effort is particularly indicated when the starter clicks

instead of cranking. After a few times around you should

also take the starter motor apart, check the internal wiring

and the brushes. Clean the commutator with about 240

grit if it is smooth but dark. Otherwise you may need to

take it to a shop and get it turned (if it has grooves in

it or is badly pitted). You will probably find that starters

resist disassembly pretty strongly, especially the big

flathead screws in the side. A normal screwdriver

will usually not do it. I generally use a 3# hammer and

a punch at one end of the slot to persuade them.

Allen head flathead screws would be a really great

replacement item here. They are metric, unlike most

of the rest of the parts on my oldtimers.

Hope this is helpful. Don't wear nice clothes to

do this and don't try it on your wife's kitchen table!