what about the fan? thats the most important part of the cooling system (along with the waterpump). do you have a clutch fan? If so, its prolly worn out and not spinning at a high enough rpm to draw enough air through the rad.. you can get a new clutch from most auto parts for roughly $25 for the hipo one. thats what mine cost me for my old 81 firebird that orig. had a 305 chevy. If youve dont that and still have problems, then your block/heads must have some buildup or blockage in the coolant passages. youd need at the least a good power flush. firestone makes a good acid flush that will eat corrosion away but it requires a good 3-4 hours to do properly. you can get if from your local chevy/gmc dealership cause they use it on mid 90's jimmys and blazers to cure theyre corrosion problem. Im a tech. at a dealership, thats how I know. the good part about that is its only like $7.
good luck, chris
------------------
1985 Iroc Z, (parting out, email for parts)
1986 Iroc Z
355, ported camel hump heads,750 holley, 700r4, 10.5 compr., forged flat tops, solidcam .501I .510E, 4gen rear w/3:73 posi, girdle, and disc's, Edel. torker II, shift kit, 3500 stal, hedmann LT. headers, 4 gen seats, man. strg., lt1 starter, alum. w/p, msd 6al, coil, dist., mallory wires. mid 13's for now.

lots to play w/still
1995 z28,SS Hood, Iroc style spoiler, tints and black outs, 700r4,2400 act. stall speed,B&M hammer shifter, 17" budnick trilogy III wheels, 275/40 ZR17, 160 stat, man. cooling fan sw., homemaid CAI,1LE elbow, hypertech airfoil, t.b.bypass,!cat, flowmaster, !maf screen, relocated batt., 1LE front l.c.a.s', poly trans and torq.arm bushings, MSD 6AL, custom program, p&p maf, accel coil, and under drive pulley
mods that i have, just not on car yet:
msd wires, edelbrock mid. headers, edelbrock rpm catback
__________________
FOR SALE1995 z28, 8x,000 miles on car, fresh trans with shift kit, 40Kmiles on stock short block, just freshened up (some bearings, seals, cleaned, reassembled, painted), ported heads, ported intake, A4, white, 17" budnik's, graphite leather interior, tints, loaded, SS hood, "RS" spoiler
http://creimer.myphotoalbum.com/