strange rough running!!

I did some research and found similar probs, but also a little different. Have a '91 xj laredo, ho il6, man trans. I started to notice a slight "miss" occasionally during ddriving. It was soon followed by occasional VERY rough running which can always be cured by pulling over and keeping it revved at about 2000 rpm's for a few minutes, with the occasional slight miss while driving afterwards still. Wires, cap, rotor, plugs, fuel filter all new. No vaccum leaks, always had slight exhaust leak. TPS fairly new as well as newish alt and optima batt. Then I got VERY stuck in deep snow, torched a centerforce clutch trying to get out. IT sat there not running for about an hour, and when the tow showed up, started it up and holy crap it was loud, like straight pipe v-8 loud. It cleared up and believe it or not the damn thing has been running fine. Haven't driven it much since, still have to do clutch, but have gone maybe 300 km with no misses!! Whereas before it would be every 20 min or so.
So if anyone can shed some light on what might have caused this it would be greatly appreciated. I'm thinking o2 sensor, cps or maybe map sensor, as I have owned this for 7 years and havent replaced those yet. MAy long weekend is coming quick and have money invested in other things right now so testing and buying the right part the first time is key, as I have found throwing money at probs to be my least favorite pass time. Thanx again for any help. This site is by far the most useful tool someone could have and am DAMN PROUD to live and play in an XJ!! ( hey I like that)

this is kinda a WAG, but could it have been a sticking exhust valve? or a bad rocker? :dunno:
how many miles does it have on it

may have been, how do i check?? as for miles, ready for this, 462,000 km's!!! other than this prob, has run very well, and is maintained as much as poss. thanx.

yeah 287,000 is a lot of miles, got 350,000+ on my 89. :yelclap:
thought about the valve because of the unusually load exhaust, if the valve is stuck open you'll get detonation through your pipes and wa-la that v-8 straight pipe sound. if anyone thinks I'm out in left field somewhere, please let me know I'm still learning this stuff :read:. Probably will be till my XJ falls apart( where it can't be recovered) .