Cold or Damp cranks & backfires hard starting

92 Grand Prix w 3.1 v6. Car has 150K on odometer which is stuck and does not register miles - I've had the car since early spring and I love it! put oxy sensor, water pump,air cleaner, fresh oil and filter and inner tie rod end on it and except for check engine, overtemp and low fuel lights being on - it's a great car. I don't worry about the lights cause the temp and fuel gauges work. Lately if it sits a day or more and it's cold or damp, it may refuse to start requiring a lot of cranking (2 minutes total cranking over say a half hour) for it to start. It sounds like it is trying to run and like it is loaded with fuel. When it does start it takes a little reving to get it running smoothly. Then it's just fine.
I have tried holding the gas wide open and partial, with engine cranking and when it is sitting with the key off - no difference. Yesterday it was -10 f and I had to jump it as battery was getting weak after so much cranking - while hood was up I noticed the air box lid was moving up and down when the engine made slight backfire noises - the air box cover is broken by all the mounting bosses ! I also noticed a hissing noise coming from the engine between cranking attempts. Finally it started - and has been fine since - any ideas ? It seems to be getting worse, don't know if warmer weather will "cure it" but I have been lucky and it has only refused to start in my driveway so far! Any help will be apreciated...thanks

needs a tune up with wires

I thought it might be the electronics but I pulled the plugs and it was just incredible a .125 gap! don't know much about the car except that the odometer is inop at 152k and that it has a replacement junkyard motor. A general note to owners who want to check/change plugs - the secret to gaining access to the rear 3 is to pull the alt and bracket! car runs great and started this am @ -22f.

Also 3 quick questions

1. Very slight leak at the steel line flange on top of water pump. any tricks to seal this?

2. I tried to clean and lube IAC motor to cure low curb idle it works for a while then stalls again is this a normal wear item in a hi mile car?

3. Is there a digital electronic odometer available that I could hook up to be able to keep track of miles driven on my car ?

thanks to everyone on this board - lots of good info.