How many miles are on the engine? Sorry, but 8 months at a shop? have they actively checked the obvious stuff? Has the ecu thrown any codes?
need a little deeper explanation of 'bogging and dying' too...Will the engine hold rpm after heated, but then simply stall when the throttle is released? Or does it make little to no power even with the throttle opened, and stalls regardless of what you do?
A condition of little to no engine power after fully heated can be caused by any number of things, even multiple different problems. First, how is the engine oil? Is it burning a lot of oil, leaking, etc? Did the shop perform a full compression test (static cylinder compression as well as a leak down test?). If the rings and/or piston walls are damaged or heavily worn, compression can plummet after heated...and the engine simply isn't going to work.
If the physical engine condition checks out, your problem is then most likely electrical and/or vacuum related. For the reasons i just posted; vacuum problems tend to become more noticeable after an engine is fully heated. Cylinder vacuum decreases as the iron block expands, especially at lower engine speeds...our computers are a little tricky with vacuum leaks, too...some will immediately cause a code to be thrown, some can go completely unnoticed by the ecu yet drastically hurt engine behavior...
As far as common issues that cause what you might be experiencing...The EGR valve is known to stick open on higher mileage engines...causing very poor idle and stalling issues with hot engines...after a while, the stuck open EGR will let carbon build up on the Idle Air Control valve (both of these are mounted to the throttle body, IAC on top, EGR underneath)...making the issue even worse in terms of idle and stalling, and quickly doubling the cost of repair. Both of these systems 'can' be cleaned after removal, but that is generally regarded as a band aid rather than a complete fix.
If you're having hesitation and problems after heated, outside of lower rpm, the problem could be a damaged MAF sensor, fuel delivery problems (failing pump, leaking injectors, etc.)...but these types of problems usually don't care about the engine temp...they'll cause noticeable issues no matter what temp the engine is at...
Sorry to be so vague...but its hard to give an exact answer without a lot more information...If you're sure the engine is in ok shape, and not burning tons of oil...i'd lean heavily towards a major vacuum leak or a completely gunked up EGR/IAC valve...
everything you explained is what I was going to suggest. (iagree)
