Toyota Repair: Toyota Previa Repair, power steering pump, head gasket

Question
I have 7 Previas. A '91 with 262K mi. misses after start up but smooths out after 15 secs to 1 min. Had injectors replaced (helped some-missing took 2-5 min. before)Sometimes white smoke appears at exhaust but not all times.  Mechanic suggested could be head gasket. A Lexus I had with a similar symptom turned out to be the power steering pump. Any ideas? Thanks.

Answer
The mechanic is probably correct, white smoke is steam, which is caused by coolant or water getting into the combustion chamber and getting superheated and comes out of the tailpipe as steam, these engines are also prone to cracked cylinder heads so have the head checked very thoroughly for that possibility.