1988 1.8 16V misfiring after warmed up

[RooBah]

Yes its old, but I bought it new :P
At any rate..
1988 Golf gti 16v 1.8L
175k miles
Purchased in Oregon but it has resided here in the midwest (Missouri) since 1991. I live at a private lake with dirt/gravel roads and do keep clean filters in it.
I had to put one tranny in it in 2001.
I also put a head gasket on in 1996 due to broken timing belt..yes, I learned my lesson!
Other than the dynomax muffler and opening the closed off vents in the lower airbox (south of the MAF), it is stock.

Main problem:
The car runs perfectly until it warms up (5 minutes or @ about 175°), then it acts like it is running on 2.x cyl.

Notable things:
I do not drive it often since 1999 and usualy only race it at SCCA.
The MFDisplay now seems afu.
I hit a deer a few years back and had to get the cold start valve heli-arced (was out of money after replaceing the entire front end). Has worked fine for the most part. Though, sometimes after driving it for 45 min and letting it set for 30-45 minutes, if it does not start right up, it takes minutes of cranking to get it to fire up. I was able to live with that (avoiding letting it cool down much b4 a return trip ect), that is, until this misfiring commenced.


Any help would be greatly appreciated.

Sincerely,
Roo