Hi i have a 98 Camaro thats getting a P0300 (random misfire) code but only above 75mph.. Has anyone ever had this happen to them? I dont have a nice code reader to assist in this just a cheapo one.. Any help would be appreciated. Thanks

 

One of the better scan tools, the Torque Pro app, can be bought for just $5 if you have an Android phone. The cable costs about $25 for an ELM327/OBD2 Bluetooth cable from BAFX. The software takes a bit of time and effort to set up the first time, but can display almost all engine data that is available. Unfortunately there isn't much in the way of a user manual.
Here's a photo showing a sample screen with misfire counts for all cylinders of a V6.

 

Yep +1 on above, also a scanner that displays DTC history will show you the fuel trims at the DTC timeframe. Torque App will show it in real time or log. If fuel trims are above +15% you have a delivery problem. Is the miss at >70 MPH during cruising +/- 2000 RPM in 4th or during a take off in 2nd or 3rd at higher RPM and load? the P0300 means PCM unable to determine failing cylinder OR multiple cylinders. Usually has to have quite a few missfires before the code is set (something like 50 or more) so the counts Gary pointed out would show whats going on. Not the cause, but identifies the suspect cylinders.

 

The Torque app can also display freeze frame info for the DTC's.