Bought a 1993 UK twin turbo 2 months ago. With re-built engine & new performance downpipe as it's only mod. Everything else is standard.

79k on clock.

Engine compression shows @115.

Car's been running great until today.

The car now has very bad exhaust blow back, like a spluttering, choking backfire (no bangs, just putting sounds). Engine idle needle stays stantionary at @800.
Engine looks to be steady no juttering, but the exhaust and breathing of the car is spluttering/juttering very badly.

When I put the foot on the gas, it feels that it lacks power or it isn't firing correctly. When I take my foot of the peddle it stalls.

Has anybody got any suggestions to what could be the cause of such symptoms & what to check?