89 4.3 S10 Blazer overheat problem

I have replaced the water pump and thermostat. Does not leak anymore but after running fine for a while it starts to get hotter (250 or more). I don't seem to be losing fluid or getting antfreeze in the oil or oil in the antifreeze so I don't think it could be a head gasket issue. I have not had the radiator tested but I'm thinking that may be the next step. Any ideas would be appreciated. Thanks.

Does the rad boil over?
Are you sure the tempt gauge is right?
Does it heat up sitting still or on the road or going up a hill?

It does not boil over but I don't know if the gauge is accurate either. I have not driven it enough to see if it only gets hot under load. I'll try to get some better symptoms and post them back later.

Also bleed the coolant system you may have a air lock.
If it is getting real hot you will have a boil over.
Double check you tempt gauge.

Can you tell me how to bleed the cooling system? I do have a chiltons manual for the vehicle so maybe the info is in there? They didn't talk about doing this in the instructions for changing the water pump? Thanks.

I usually remove a heater hose and let air bleed out.

Thanks for the advice....so I went home last night and started it up and removed the radiator cap thinking this should definately let the air bleed out. It sat there and ran falwlessly for 45 minutes at 195 degrees. I could not see fluid flowing in the neck of the radiator but it did bounce up and down maybe a 1/4". The upper hose got hot so I'm thinking the thermostat must be functioning properly. I even cranked the air conditioning on max for 20 minutes to see it something there was causing it to overheat...no change...temp held steady as a rock. Turned off the engine and went to put the radiator cap back on and what do I find but coolant gushing out of the radiator....what the heck would cause that??? If it was going to overflow wouldn't it do so while the engine was running?