Firebird - 1991 - misc. problems

[julesfb91]

A couple of years ago i bought a 1991 firebird for $600... all I could afford at time. Problems it had: knock in engine / AC, radio, cigarette lighter, windshield wipers non-working.
I replaced engine December 2002 (at 185,000 miles.. for $3000). Shortly after I had to keep putting in antifreeze... then the head gasket blew... was replaced under warranty of engine (now I'm wondering what kind of damage was done to engine while it was driven a few days before gasket blew completely.
I assumed they changed the oil again (as coolant/water got in it) then, but now I'm not sure. From that I waited to change the oil for another 4000 miles. So, if they didn't change the oil, then it may hve been over 7000 miles between changes. I'm wondering what damage could've been done to engine if that's the case.
I was told by Oil Can Henry guys that I possibly had leaks in front main seal, oil pan, and rear differential--- suggesting that whoever replaced the engine did not adjust things correctly.
NOW... OIL PRESSURE GUAGE has been inconsistent and getting worse (even tho oil level is correct when I check it- everyday)... when I start out, the guage reads between 40 & 50, then the longer I drive the lower it gets (down to 15-20 and when idling at stop signs - to zero, going back up to 15-30 upon accelerating.
AND--- recently it started sounding like one or both mufflers are going to heck... don't see or feel any damage, holes, etc.
AND- the drivers side electric window goes down fine, but goes up only 1/4" every 60 seconds or so... other widow works fine.
-- any ideas on what is wrong, how much it'll cost, etc would be helpful, so I am armed with info when I take it back to the shop (anyone know of reputable shops in Portland, Oregon?)
-- and with the minor problems (wipers, AC, cigarette lighter), any help would be welcome, as I'll get them fixed when I can afford it.

thanxz much for any and all help!!!

[petereggy]

the mufflers could be rusting from the inside so you can replace those with good flowmasters for about 40-50 bucks each your power window going could need a new motor i dont know how much those are

and with your ac and so on it could be as simple as fuses but you could get it checked from an auto electricion