I have a 2000 chevy blazer ZR2
about two weeks ago while on the hyway at about 65 mph the engine start to misfire bad then I heard a noise from the engine. I stopped and found that my pulley tensioner had went and the serpentine belt was shot, which also ended up cutting through the "T" connector for the hoses from the secondary injection system, so I replaced that and resecured the hoses, along with replacing the belt and tensioner.

But the engine was still misfiring, but this time it start after driving for a few minutes and around 40 mph. the engine light was on and I had the P0300 code and a code for the catalytic converter, which was also making a bad rattling noise. Come to find out the honeycombs had come loose and were blocking the hole for the exhaust, restricting the flow through the Cat., the cat has been replaced, but i still get misfires now only at hyway speeds.

also, I was getting a code for the secondary air injection system, found the fuse blown, replaced that, but it keeps on blowing, although it is not tripping the code for the engine light.

I have replaced all the plugs, wires, distributor cap and rotor. I also sprayed MAF cleaner on the MAF sensor, that seemed to help for a day, but then it started to misfire again, although not as bad. I also every ow and then get a code P0440. but after spraying the MAF it hasnt come back. The only code that keeps coming up is P0300

My engine code reader gave the following in the freeze frame
ABSLT - 34.1 %
engine rpm 2251
calc load 27.4
MAF - 47.1
MAP - 80 kpa
coolant 194deg
st ftrm1 -7.8%
lt ftrm1 _ 21.0%
st ftrm2 - 14.0
lt ftrm2 - 23.4
fuel sys 1 & 2 - clsd
speed - 68 mph

a check of the I/M monitors showed all applicable montiors were ready except the EVAP SYS was not ready

Any ideas on what is causing the misfiring?
it runs smooth at idle and driving around town, its only when i get on the hyway and have been travelling at about 2500 rpm (60-65 mph) for a few minutes.