Like above mentioned I'd try a new 15 to 16 psi cap first -thats a cheap fix.
Other thoughts....
I'd go with a bigger raditor if the rules allow, you might also consider a aluminum one if you can spare the change, they really make a difference.
Also a aluminum high flow water pump will make a big difference too - they can be had for around 50 bucks just about anywhere, I got mine from a vendor on here.
Goodluck, Charles